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Table of Contents

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1 Introduction

The purpose of virtio and this specification is that virtual environments and guests should have a straightforward, efficient, standard and extensible mechanism for virtual devices, rather than boutique per-environment or per-OS mechanisms.

Straightforward: Virtio devices use normal bus mechanisms of interrupts and DMA which should be familiar to any device driver author. There is no exotic page-flipping or COW mechanism: it’s just a normal device. ¹

Efficient: Virtio devices consist of rings of descriptors for input and output, which are neatly separated to avoid cache effects from both driver and device writing to the same cache lines.

Standard: Virtio makes no assumptions about the environment in which it operates, beyond supporting the bus attaching the device. Virtio devices are implemented over PCI and other buses, and earlier drafts been implemented on other buses not included in this spec. ²

Extensible: Virtio PCI devices contain feature bits which are acknowledged by the guest operating system during device setup. This allows forwards and backwards compatibility: the device offers all the features it knows about, and the driver acknowledges those it understands and wishes to use.

1.1 Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

1.2 Normative References

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2 Basic Facilities of a Virtio Device

• Device Status field
• Feature bits
• Configuration space
• One or more virtqueues

Unless explicitly specified otherwise, all multi-byte fields are little-endian. To reinforce this the examples use typenames like "le16" instead of "uint16_t".

2.1 Device Status Field

The driver MUST update the Device Status field in the order below to indicate its progress. This provides a simple low-level diagnostic: it’s most useful to imagine them hooked up to traffic lights on the console indicating the status of each device. The driver MUST NOT clear a device status bit.

This field is 0 upon reset, otherwise at least one bit should be set:

ACKNOWLEDGE (1)

Indicates that the guest OS has found the device and recognized it as a valid virtio device.

DRIVER (2)

Indicates that the guest OS knows how to drive the device. Under Linux, drivers can be loadable modules so there may be a significant (or infinite) delay before setting this bit.

FEATURES_OK (8)

Indicates that the driver has acknowledged all the features it understands, and feature negotiation is complete.

DRIVER_OK (4)

Indicates that the driver is set up and ready to drive the device.

FAILED (128)

Indicates that something went wrong in the guest, and it has given up on the device. This could be an internal error, or the driver didn’t like the device for some reason, or even a fatal error during device operation. The driver MUST reset the device before attempting to re-initialize.

2.2 Feature Bits

Each virtio device offers all the features it understands. During device initialization, the driver reads this and tells the device the subset that it accepts. The only way to renegotiate is to reset the device.

This allows for forwards and backwards compatibility: if the device is enhanced with a new feature bit, older drivers will not write that feature bit back to the device and it SHOULD go into backwards compatibility mode. Similarly, if a driver is enhanced with a feature that the device doesn’t support, it see the new feature is not offered and SHOULD go into backwards compatibility mode (or, for poor implementations it MAY set the FAILED Device Status bit).

The driver MUST NOT accept a feature which the device did not offer, and MUST NOT accept a feature which requires another feature which was not accepted.

The device MUST NOT offer a feature which requires another feature which was not offered.

Feature bits are allocated as follows:

0 to 23

Feature bits for the specific device type

24 to 32

Feature bits reserved for extensions to the queue and feature negotiation mechanisms

33 and above

Feature bits reserved for future extensions.

For example, feature bit 0 for a network device (i.e. Subsystem Device ID 1) indicates that the device supports checksumming of packets.

In particular, new fields in the device configuration space are indicated by offering a feature bit, so the driver MUST check that the feature is offered before accessing that part of the configuration space.

2.2.1 Legacy Interface: A Note on transitions from earlier drafts

Earlier drafts of this specification (up to 0.9.X) defined a similar, but different interface between the hypervisor and the guest. Since these are widely deployed, this specification accommodates optional features to simplify transition from these earlier draft interfaces. Specifically:

Legacy Interface

is an interface specified by an earlier draft of this specification (up to 0.9.X)

Legacy Device

is a device implemented before this specification was released, and implementing a legacy interface on the host side

Legacy Driver

is a driver implemented before this specification was released, and implementing a legacy interface on the guest side

Legacy devices and legacy drivers are not compliant with this specification.

To simplify transition from these earlier draft interfaces, it is possible to implement:

Transitional Device

a device supporting both drivers conforming to this specification, and allowing legacy drivers.

Transitional Driver

a driver supporting both devices conforming to this specification, and legacy devices.

Transitional devices and transitional drivers can be compliant with this specification (ie. when not operating in legacy mode).

Devices or drivers with no legacy compatibility are referred to as non-transitional devices and drivers, respectively.

Transitional Drivers can detect Legacy Devices by detecting that the feature bit VIRTIO_F_VERSION_1 is not offered. Transitional devices can detect Legacy drivers by detecting that VIRTIO_F_VERSION_1 has not been acknowledged by the driver. In this case device is used through the legacy interface.

To make them easier to locate, specification sections documenting these transitional features are explicitly marked with ’Legacy Interface’ in the section title.

2.3 Configuration Space

Configuration space is generally used for rarely-changing or initialization-time parameters. Drivers MUST NOT assume reads from fields greater than 32 bits wide are atomic, nor or reads from multiple fields.

Each transport provides a generation count for the configuration space, which must change whenever there is a possibility that two accesses to the configuration space can see different versions of that space.

Thus drivers SHOULD read configuration space fields like so:

Note that configuration space uses the little-endian format for multi-byte fields.

Note that future versions of this specification will likely extend the configuration space for devices by adding extra fields at the tail end of some structures in configuration space.

To allow forward compatibility with such extensions, drivers MUST NOT limit structure size and configuration space size. Instead, drivers SHOULD only check that configuration space is *large enough* to contain the fields required for device operation.

For example, if the specification states that configuration space ’includes a single 8-bit field’ drivers should understand this to mean that the configuration space might also include an arbitrary amount of tail padding, and accept any configuration space size equal to or greater than the specified 8-bit size.

2.3.1 Legacy Interface: A Note on Configuration Space endian-ness

Note that for legacy interfaces, configuration space is generally the guest’s native endian, rather than PCI’s little-endian.

2.3.2 Legacy Interface: Configuration Space

Legacy devices did not have a configuration generation field, thus are susceptible to race conditions if configuration is updated. This effects the block capacity and network mac fields; best practice is to read these fields multiple times until two reads generate a consistent result.

2.4 Virtqueues

The mechanism for bulk data transport on virtio devices is pretentiously called a virtqueue. Each device can have zero or more virtqueues: for example, the simplest network device has one for transmit and one for receive. Each queue has a 16-bit queue size parameter, which sets the number of entries and implies the total size of the queue.

Each virtqueue consists of three parts:

• Descriptor Table
• Available Ring
• Used Ring

where each part is physically-contiguous in guest memory, and has different alignment requirements.

The memory aligment and size requirements, in bytes, of each part of the virtqueue are summarized in the following table:

The Alignment column gives the miminum alignment: for each part of the virtqueue, the physical address of the first byte MUST be a multiple of the specified alignment value.

The Size column gives the total number of bytes required for each part of the virtqueue.

Queue Size corresponds to the maximum number of buffers in the virtqueue. For example, if Queue Size is 4 then at most 4 buffers can be queued at any given time. Queue Size value is always a power of 2. The maximum Queue Size value is 32768. This value is specified in a bus-specific way.

When the driver wants to send a buffer to the device, it fills in a slot in the descriptor table (or chains several together), and writes the descriptor index into the available ring. It then notifies the device. When the device has finished a buffer, it writes the descriptor index into the used ring, and sends an interrupt.

2.4.1 Legacy Interfaces: A Note on Virtqueue Layout

For Legacy Interfaces, several additional restrictions are placed on the virtqueue layout:

Each virtqueue occupies two or more physically-contiguous pages (usually defined as 4096 bytes, but depending on the transport) and consists of three parts:

The bus-specific Queue Size field controls the total number of bytes required for the virtqueue according to the following formula:

This wastes some space with padding. The legacy virtqueue layout structure therefore looks like this:

2.4.2 Legacy Interfaces: A Note on Virtqueue Endianness

Note that the endian of fields and in the virtqueue is the native endian of the guest, not little-endian as specified by this standard. It is assumed that the host is already aware of the guest endian.

2.4.3 Message Framing

The device MUST NOT make assumptions about the particular arrangement of descriptors: the message framing is independent of the contents of the buffers. For example, a network transmit buffer consists of a 12 byte header followed by the network packet. This could be most simply placed in the descriptor table as a 12 byte output descriptor followed by a 1514 byte output descriptor, but it could also consist of a single 1526 byte output descriptor in the case where the header and packet are adjacent, or even three or more descriptors (possibly with loss of efficiency in that case).

Note that, some implementations may have large-but-reasonable restrictions on total descriptor size (such as based on IOV_MAX in the host OS). This has not been a problem in practice: little sympathy will be given to drivers which create unreasonably-sized descriptors such as by dividing a network packet into 1500 single-byte descriptors!

2.4.3.1 Legacy Interface: Message Framing

Regrettably, initial driver implementations used simple layouts, and devices came to rely on it, despite this specification wording. In addition, the specification for virtio_blk SCSI commands required intuiting field lengths from frame boundaries (see 5.2.5.1 Legacy Interface: Device Operation)

It is thus recommended that when using legacy interfaces, transitional drivers be conservative in their assumptions, unless the VIRTIO_F_ANY_LAYOUT feature is accepted.

2.4.4 The Virtqueue Descriptor Table

The descriptor table refers to the buffers the driver is using for the device. The addresses are physical addresses, and the buffers can be chained via the next field. Each descriptor describes a buffer which is read-only or write-only, but a chain of descriptors can contain both read-only and write-only buffers.

The actual contents of the memory offered to the device depends on the device type. Most common is to begin the data with a header (containing little-endian fields) for the device to read, and postfix it with a status tailer for the device to write.

Drivers MUST NOT add a descriptor chain over than 2³² bytes long in total; this implies that loops in the descriptor chain are forbidden!

The number of descriptors in the table is defined by the queue size for this virtqueue.

2.4.4.1 Indirect Descriptors

Some devices benefit by concurrently dispatching a large number of large requests. The VIRTIO_RING_F_INDIRECT_DESC feature allows this (see 7 virtio_ring.h). To increase ring capacity the driver can store a table of indirect descriptors anywhere in memory, and insert a descriptor in main virtqueue (with flags&VRING_DESC_F_INDIRECT on) that refers to memory buffer containing this indirect descriptor table; fields addr and len refer to the indirect table address and length in bytes, respectively.

The driver MUST NOT set the VRING_DESC_F_INDIRECT flag unless the VIRTIO_RING_F_INDIRECT_DESC feature was negotiated.

The indirect table layout structure looks like this (len is the length of the descriptor that refers to this table, which is a variable, so this code won’t compile):

The first indirect descriptor is located at start of the indirect descriptor table (index 0), additional indirect descriptors are chained by next field. An indirect descriptor without next field (with flags&VRING_DESC_F_NEXT off) signals the end of the descriptor. An indirect descriptor can not refer to another indirect descriptor table (flags&VRING_DESC_F_INDIRECT MUST be off). A single indirect descriptor table can include both read-only and write-only descriptors; the device MUST ignore the write-only flag (flags&VRING_DESC_F_WRITE) in the descriptor that refers to it.

2.4.5 The Virtqueue Available Ring

The available ring refers to what descriptor chains the driver is offering the device: each ring entry refers to the head of a descriptor chain. It is only written by the driver and read by the device.

The “idx” field indicates where we would put the next descriptor entry in the ring (modulo the queue size). This starts at 0, and increases.

If the VIRTIO_RING_F_INDIRECT_DESC feature bit is not negotiated, the “flags” field offers a crude interrupt control mechanism. The driver MUST set this to 0 or 1: 1 indicates that the device SHOULD NOT send an interrupt when it consumes a descriptor chain from the available ring. The device MUST ignore the used_event value in this case.

Otherwise, if the VIRTIO_RING_F_EVENT_IDX feature bit is negotiated, the driver MUST set the "flags" field to 0, and use the “used_event” field in the used ring instead. The driver can ask the device to delay interrupts until an entry with an index specified by the “used_event” field is written in the used ring (equivalently, until the idx field in the used ring will reach the value used_event + 1).

The driver MUST handle spurious interrupts: either form of interrupt suppression is merely an optimization; it may not suppress interrupts entirely.

2.4.6 The Virtqueue Used Ring

The used ring is where the device returns buffers once it is done with them: it is only written to by the device, and read by the driver.

Each entry in the ring is a pair: the head entry of the descriptor chain describing the buffer (this matches an entry placed in the available ring by the guest earlier), and the total of bytes written into the buffer. The latter is extremely useful for drivers using untrusted buffers: if you do not know exactly how much has been written by the device, you usually have to zero the buffer to ensure no data leakage occurs.

If the VIRTIO_RING_F_INDIRECT_DESC feature bit is not negotiated, the “flags” field offers a crude interrupt control mechanism. The driver MUST initialize this to 0, the device MUST set this to 0 or 1: 1 indicates that the driver SHOULD NOT send an notification when it adds a descriptor chain to the available ring. The driver MUST ignore the used_event value in this case.

Otherwise, if the VIRTIO_RING_F_EVENT_IDX feature bit is negotiated, the device MUST leave the "flags" field at 0, and use the “avail_event” field in the used ring instead. The device can ask the driver to delay notifications until an entry with an index specified by the “avail_event” field is written in the available ring (equivalently, until the idx field in the used ring will reach the value avail_event + 1).

The device MUST handle spurious notification: either form of notification suppression is merely an optimization; it may not suppress them entirely.

2.4.7 Helpers for Operating Virtqueues

The Linux Kernel Source code contains the definitions above and helper routines in a more usable form, in include/linux/virtio_ring.h. This was explicitly licensed by IBM and Red Hat under the (3-clause) BSD license so that it can be freely used by all other projects, and is reproduced (with slight variation to remove Linux assumptions) in 7 virtio_ring.h.

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3 General Initialization And Device Operation

3.1 Device Initialization

The driver MUST follow this sequence to initialize a device:

1. Reset the device.
2. Set the ACKNOWLEDGE status bit: we have noticed the device.
3. Set the DRIVER status bit: we know how to drive the device.
4. Read device feature bits, and write the subset of feature bits understood by the OS and driver to the device.
5. Set the FEATURES_OK status bit. The driver MUST not accept new feature bits after this step.
6. Re-read the status byte to ensure the FEATURES_OK bit is still set: otherwise, the device does not support our subset of features and the device is unusable.
7. Perform device-specific setup, including discovery of virtqueues for the device, optional per-bus setup, reading and possibly writing the device’s virtio configuration space, and population of virtqueues.
8. Set the DRIVER_OK status bit. At this point the device is "live".

If any of these steps go irrecoverably wrong, the driver SHOULD set the FAILED status bit to indicate that it has given up on the device (it can reset the device later to restart if desired). The driver MUST not continue initialization in that case.

The device MUST NOT consume buffers before DRIVER_OK, and the driver MUST NOT notify the device before it sets DRIVER_OK.

Devices SHOULD support all valid combinations of features, but we know that implementations may well make assuptions that they will only be used by fully-optimized drivers. The resetting of the FEATURES_OK flag provides a semi-graceful failure mode for this case.

3.1.1 Legacy Interface: Device Initialization

Legacy devices do not support the FEATURES_OK status bit, and thus did not have a graceful way for the device to indicate unsupported feature combinations. It also did not provide a clear mechanism to end feature negotiation, which meant that devices finalized features on first-use, and no features could be introduced which radically changed the initial operation of the device.

Legacy device implementations often used the device before setting the DRIVER_OK bit.

The result was the steps 5 and 6 were omitted, and steps 7 and 8 were conflated.

3.2 Device Operation

There are two parts to device operation: supplying new buffers to the device, and processing used buffers from the device. As an example, the simplest virtio network device has two virtqueues: the transmit virtqueue and the receive virtqueue. The driver adds outgoing (read-only) packets to the transmit virtqueue, and then frees them after they are used. Similarly, incoming (write-only) buffers are added to the receive virtqueue, and processed after they are used.

3.2.1 Supplying Buffers to The Device

The driver offers buffers to one of the device’s virtqueues as follows:

1. The driver places the buffer into free descriptor(s) in the descriptor table, chaining as necessary (see 2.4.4 The Virtqueue Descriptor Table).
2. The driver places the index of the head of the descriptor chain into the next ring entry of the available ring.
3. Steps 1 and 2 may be performed repeatedly if batching is possible.
4. The driver MUST perform suitable a memory barrier to ensure the device sees the updated descriptor table and available ring before the next step.
5. The available “idx” field is increased by the number of descriptor chain heads added to the available ring.
6. The driver MUST perform a suitable memory barrier to ensure that it updates the "idx" field before checking for notification suppression.
7. If notifications are not suppressed, the driver MUST notify the device of the new available buffers.

Note that the above code does not take precautions against the available ring buffer wrapping around: this is not possible since the ring buffer is the same size as the descriptor table, so step (1) will prevent such a condition.

In addition, the maximum queue size is 32768 (it must be a power of 2 which fits in 16 bits), so the 16-bit “idx” value can always distinguish between a full and empty buffer.

Here is a description of each stage in more detail.

3.2.1.1 Placing Buffers Into The Descriptor Table

A buffer consists of zero or more read-only physically-contiguous elements followed by zero or more physically-contiguous write-only elements (it must have at least one element). This algorithm maps it into the descriptor table to form a descriptor chain:

for each buffer element, b:

1. Get the next free descriptor table entry, d
2. Set d.addr to the physical address of the start of b
3. Set d.len to the length of b.
4. If b is write-only, set d.flags to VRING_DESC_F_WRITE, otherwise 0.
5. If there is a buffer element after this:
   1. Set d.next to the index of the next free descriptor element.
   2. Set the VRING_DESC_F_NEXT bit in d.flags.

In practice, the d.next fields are usually used to chain free descriptors, and a separate count kept to check there are enough free descriptors before beginning the mappings.

3.2.1.2 Updating The Available Ring

The head of the buffer we mapped is the first d in the algorithm above (the descriptor chain head). A naive implementation would do the following (with the appropriate conversion to-and-from little-endian assumed):

However, in general we can add many descriptor chains before we update the “idx” field (at which point they become visible to the device), so we keep a counter of how many we’ve added:

3.2.1.3 Updating The Index Field

Once the index field of the virtqueue is updated, the device will be able to access the descriptor chains we’ve created and the memory they refer to. This is why a memory barrier is generally used before the index update, to ensure it sees the most up-to-date copy.

The index field always increments, and we let it wrap naturally at 65536:

3.2.1.4 Notifying The Device

The actual method of device notification is bus-specific, but generally it can be expensive. So the device can suppress such notifications if it doesn’t need them. The driver has to be careful to expose the new index value before checking if notifications are suppressed: it’s OK to notify gratuitously, but not to omit a required notification. So again, we use a memory barrier here before reading the flags or the avail_event field.

If the VIRTIO_F_RING_EVENT_IDX feature is not negotiated, and if the VRING_USED_F_NOTIFY flag is not set, we go ahead and notify the device.

If the VIRTIO_F_RING_EVENT_IDX feature is negotiated, we read the avail_event field in the available ring structure. If the available index crossed_the avail_event field value since the last notification, we go ahead and write to the PCI configuration space. The avail_event field wraps naturally at 65536 as well, iving the following algorithm for calculating whether a device needs notification:

3.2.2 Receiving Used Buffers From The Device

Once the device has used a buffer (read from or written to it, or parts of both, depending on the nature of the virtqueue and the device), it sends an interrupt, following an algorithm very similar to the algorithm used for the driver to send the device a buffer:

1. Write the head descriptor number to the next field in the used ring.
2. Update the used ring index.
3. Deliver an interrupt if necessary:
   1. If the VIRTIO_F_RING_EVENT_IDX feature is not negotiated: check if the VRING_AVAIL_F_NO_INTERRUPT flag is not set in avail->flags.
   2. If the VIRTIO_F_RING_EVENT_IDX feature is negotiated: check whether the used index crossed the used_event field value since the last update. The used_event field wraps naturally at 65536 as well:
        (u16)(new_idx - used_event - 1) < (u16)(new_idx - old_idx)

For each ring, the driver should then disable interrupts by writing VRING_AVAIL_F_NO_INTERRUPT flag in avail structure, if required. It can then process used ring entries finally enabling interrupts by clearing the VRING_AVAIL_F_NO_INTERRUPT flag or updating the EVENT_IDX field in the available structure. The driver should then execute a memory barrier, and then recheck the ring empty condition. This is necessary to handle the case where after the last check and before enabling interrupts, an interrupt has been suppressed by the device:

3.2.3 Notification of Device Configuration Changes

For devices where the configuration information can be changed, an interrupt is delivered when a configuration change occurs.

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4 Virtio Transport Options

4.1 Virtio Over PCI Bus

Virtio devices are commonly implemented as PCI devices.

4.1.1 PCI Device Discovery

Any PCI device with Vendor ID 0x1AF4, and Device ID 0x1000 through 0x103F inclusive is a virtio device³ .

The Subsystem Device ID indicates which virtio device is supported by the device. The Subsystem Vendor ID SHOULD reflect the PCI Vendor ID of the environment (it’s currently only used for informational purposes by the driver).

All drivers MUST match devices with any Revision ID, this is to allow devices to be versioned without breaking drivers.

4.1.1.1 Legacy Interfaces: A Note on PCI Device Discovery

Transitional devices must have a Revision ID of 0 to match legacy drivers.

Non-transitional devices must have a Revision ID of 1 or higher.

Both transitional and non-transitional drivers must match any Revision ID value.

4.1.2 PCI Device Layout

To configure the device, use I/O and/or memory regions and/or PCI configuration space of the PCI device. These contain the virtio header registers, the notification register, the ISR status register and device specific registers, as specified by Virtio Structure PCI Capabilities.

There may be different widths of accesses to the I/O region; the “natural” access method for each field must be used (i.e. 32-bit accesses for 32-bit fields, etc).

PCI Device Configuration Layout includes the common configuration, ISR, notification and device specific configuration structures.

All multi-byte fields are little-endian.

4.1.2.1 Common configuration structure layout

Common configuration structure layout is documented below:

device_feature_select

The driver uses this to select which Feature Bits the device_feature field shows. Value 0x0 selects Feature Bits 0 to 31 Value 0x1 selects Feature Bits 32 to 63 The device MUST present 0 on device_feature for any other value.

device_feature

The device uses this to report Feature Bits to the driver. Device Feature Bits selected by device_feature_select.

driver_feature_select

The driver uses this to select which Feature Bits the driver_feature field shows. Value 0x0 selects Feature Bits 0 to 31 Value 0x1 selects Feature Bits 32 to 63 When set to any other value, reads from driver_feature return 0, writing 0 into driver_feature has no effect. The driver MUST not write any other value into driver_feature (a corollary of the rule that the driver can only write a subset of device features).

driver_feature

The driver writes this to accept feature bits offered by the device. Driver Feature Bits selected by driver_feature_select.

msix_config

The driver sets the Configuration Vector for MSI-X.

num_queues

The device specifies the maximum number of virtqueues supported here.

device_status

The driver writes the Device Status here. Writing 0 into this field resets the device.

config_generation

Configuration atomicity value. The device changes this every time the configuration noticeably changes. This means the device may only change the value after a configuration read operation, but MUST change it if there is any risk of a driver seeing an inconsistent configuration state.

queue_select

Queue Select. The driver selects which virtqueue the following fields refer to.

queue_size

Queue Size. On reset, specifies the maximum queue size supported by the hypervisor. This can be modified by driver to reduce memory requirements. The device MUST set this to 0 if this virtqueue is unavailable.

queue_msix_vector

The driver uses this to specify the Queue Vector for MSI-X.

queue_enable

The driver uses this to selectively prevent the device from executing requests from this virtqueue. 1 - enabled; 0 - disabled

The driver MUST configure the other virtqueue fields before enabling the virtqueue.

queue_notify_off

The driver reads this to calculate the offset from start of Notification structure at which this virtqueue is located. Note: this is *not* an offset in bytes. See notify_off_multiplier below.

queue_desc

The driver writes the physical address of Descriptor Table here.

queue_avail

The driver writes the physical address of Available Ring here.

queue_used

The driver writes the physical address of Used Ring here.

4.1.2.2 ISR status structure layout

ISR status structure includes a single 8-bit ISR status field.

4.1.2.3 Notification structure layout

Notification structure is always a multiple of 2 bytes in size. It includes 2-byte Queue Notify fields for each virtqueue of the device. Note that multiple virtqueues can use the same Queue Notify field, if necessary: see notify_off_multiplier below.

4.1.2.4 Device specific structure

Device specific structure is optional.

4.1.2.5 Legacy Interfaces: A Note on PCI Device Layout

Transitional devices should present part of configuration registers in a legacy configuration structure in BAR0 in the first I/O region of the PCI device, as documented below.

There may be different widths of accesses to the I/O region; the “natural” access method for each field in the virtio header must be used (i.e. 32-bit accesses for 32-bit fields, etc), but when accessed through the legacy interface the device-specific region can be accessed using any width accesses, and should obtain the same results.

Note that this is possible because while the virtio header is PCI (i.e. little) endian, when using the legacy interface the device-specific region is encoded in the native endian of the guest (where such distinction is applicable).

When used through the legacy interface, the virtio header looks as follows:

If MSI-X is enabled for the device, two additional fields immediately follow this header:

Note: When MSI-X capability is enabled, device specific configuration starts at byte offset 24 in virtio header structure. When MSI-X capability is not enabled, device specific configuration starts at byte offset 20 in virtio header. ie. once you enable MSI-X on the device, the other fields move. If you turn it off again, they move back!

Immediately following these general headers, there may be device-specific headers:

Note that only Feature Bits 0 to 31 are accessible through the Legacy Interface. When used through the Legacy Interface, Transitional Devices must assume that Feature Bits 32 to 63 are not acknowledged by Driver.

As legacy devices had no configuration generation field, see 2.3.2 Legacy Interface: Configuration Space for workarounds.

4.1.3 PCI-specific Initialization And Device Operation

4.1.3.1 Device Initialization

This documents PCI-specific steps executed during Device Initialization. As the first step, driver must detect device configuration layout to locate configuration fields in memory, I/O or configuration space of the device.

4.1.3.1.1 Virtio Device Configuration Layout Detection

Virtio Device Configuration Layout includes virtio configuration header, Notification and ISR Status and device configuration structures. Each structure can be mapped by a Base Address register (BAR) belonging to the function, located beginning at 10h in Configuration Space, or accessed though PCI configuration space.

Actual location of each structure is specified using vendor-specific PCI capability located on capability list in PCI configuration space of the device. This virtio structure capability uses little-endian format; all bits are read-only:

This structure can optionally followed by extra data, depending on other fields, as documented below.

Note that future versions of this specification will likely extend devices by adding extra fields at the tail end of some structures.

To allow forward compatibility with such extensions, drivers must not limit structure size. Instead, drivers should only check that structures are *large enough* to contain the fields required for device operation.

For example, if the specification states ’structure includes a single 8-bit field’ drivers should understand this to mean that the structure can also include an arbitrary amount of tail padding, and accept any structure size equal to or greater than the specified 8-bit size.

The fields are interpreted as follows:

cap_vndr

0x09; Identifies a vendor-specific capability.

cap_next

Link to next capability in the capability list in the configuration space.

cap_len

Length of the capability structure, including the whole of struct virtio_pci_cap, and extra data if any. This length might include padding, or fields unused by the driver.

cfg_type

identifies the structure, according to the following table.

  /* Common configuration */ 
  #define VIRTIO_PCI_CAP_COMMON_CFG1 
  /* Notifications */ 
  #define VIRTIO_PCI_CAP_NOTIFY_CFG2 
  /* ISR Status */ 
  #define VIRTIO_PCI_CAP_ISR_CFG3 
  /* Device specific configuration */ 
  #define VIRTIO_PCI_CAP_DEVICE_CFG4 
  /* PCI configuration access */ 
  #define VIRTIO_PCI_CAP_PCI_CFG5

Any other value - reserved for future use. Drivers MUST ignore any vendor-specific capability structure which has a reserved cfg_type value.

More than one capability can identify the same structure - this makes it possible for the device to expose multiple interfaces to drivers. The order of the capabilities in the capability list specifies the order of preference suggested by the device; drivers SHOULD use the first interface that they can support. For example, on some hypervisors, notifications using IO accesses are faster than memory accesses. In this case, hypervisor can expose two capabilities with cfg_type set to VIRTIO_PCI_CAP_NOTIFY_CFG: the first one addressing an I/O BAR, the second one addressing a memory BAR. Driver will use the I/O BAR if I/O resources are available, and fall back on memory BAR when I/O resources are unavailable.

bar

values 0x0 to 0x5 specify a Base Address register (BAR) belonging to the function located beginning at 10h in Configuration Space and used to map the structure into Memory or I/O Space. The BAR is permitted to be either 32-bit or 64-bit, it can map Memory Space or I/O Space.

Any other value is reserved for future use. Drivers MUST ignore any vendor-specific capability structure which has a reserved bar value.

offset

indicates where the structure begins relative to the base address associated with the BAR.

length

indicates the length of the structure. This size might include padding, or fields unused by the driver. Drivers SHOULD only map part of configuration structure large enough for device operation. For example, a future device might present a large structure size of several MBytes. As current devices never utilize structures larger than 4KBytes in size, driver can limit the mapped structure size to e.g. 4KBytes to allow forward compatibility with such devices without loss of functionality and without wasting resources.

If cfg_type is VIRTIO_PCI_CAP_NOTIFY_CFG this structure is immediately followed by additional fields:

notify_off_multiplier

Virtqueue offset multiplier, in bytes. Must be even and either a power of two, or 0. Value 0x1 is reserved. For a given virtqueue, the address to use for notifications is calculated as follows:

queue_notify_off * notify_off_multiplier + offset

If notify_off_multiplier is 0, all virtqueues use the same address in the Notifications structure!

If cfg_type is VIRTIO_PCI_CAP_PCI_CFG the fields bar, offset and length are RW and this structure is immediately followed by an additional field:

pci_cfg_data

This RW field allows an indirect access to any BAR on the device using PCI configuration accesses.

The BAR to access is selected using the bar field. The length of the access is specified by the length field, which can be set to 1, 2 and 4. The offset within the BAR is specified by the offset field, which must be aligned to length bytes.

After this field is written by driver, the first length bytes in pci_cfg_data are written at the selected offset in the selected BAR.

When this field is read by driver, length bytes at the selected offset in the selected BAR are read into pci_cfg_data.

4.1.3.1.1.1 Legacy Interface: A Note on Device Layout Detection

Legacy devices did not have the Virtio PCI Capability in their capability list.

Therefore:

Transitional devices should expose the Legacy Interface in I/O space in BAR0.

Transitional drivers should look for the Virtio PCI Capabilities on the capability list. If these are not present, driver should assume a legacy device.

Non-transitional drivers should look for the Virtio PCI Capabilities on the capability list. If these are not present, driver should assume a legacy device, and fail gracefully.

Non-transitional devices, on a platform where a legacy driver for a legacy device with the same ID might have previously existed, must take the following steps to fail gracefully when a legacy driver attempts to drive them:

1. Present an I/O BAR in BAR0, and
2. Respond to a single-byte zero write to offset 18 (corresponding to Device Status register in the legacy layout) of BAR0 by presenting zeroes on every BAR and ignoring writes.

4.1.3.1.2 Queue Vector Configuration

Writing a valid MSI-X Table entry number, 0 to 0x7FF, to one of Configuration/Queue Vector registers, maps interrupts triggered by the configuration change/selected queue events respectively to the corresponding MSI-X vector. To disable interrupts for a specific event type, unmap it by writing a special NO_VECTOR value:

Reading these registers returns vector mapped to a given event, or NO_VECTOR if unmapped. All queue and configuration change events are unmapped by default.

Note that mapping an event to vector might require allocating internal device resources, and might fail. Devices MUST report such failures by returning the NO_VECTOR value when the relevant Vector field is read. After mapping an event to vector, the driver MUST verify success by reading the Vector field value: on success, the previously written value is returned, and on failure, NO_VECTOR is returned. If a mapping failure is detected, the driver can retry mapping with fewer vectors, or disable MSI-X.

4.1.3.1.3 Virtqueue Configuration

The driver does this as follows, for each virtqueue a device has:

1. Write the virtqueue index (first queue is 0) to the Queue Select field.
2. Read the virtqueue size from the Queue Size field, which MUST be a power of 2. This controls how big the virtqueue is (see 2.4 Virtqueues). If this field is 0, the virtqueue does not exist.
3. Optionally, select a smaller virtqueue size and write it in the Queue Size field.
4. Allocate and zero Descriptor Table, Available and Used rings for the virtqueue in contiguous physical memory.
5. Optionally, if MSI-X capability is present and enabled on the device, select a vector to use to request interrupts triggered by virtqueue events. Write the MSI-X Table entry number corresponding to this vector in Queue Vector field. Read the Queue Vector field: on success, previously written value is returned; on failure, NO_VECTOR value is returned.

4.1.3.1.3.1 Legacy Interface: A Note on Virtqueue Configuration

4.1.3.2 Notifying The Device

Device notification occurs by writing the 16-bit virtqueue index of this virtqueue to the Queue Notify field.

4.1.3.3 Virtqueue Interrupts From The Device

If an interrupt is necessary:

• If MSI-X capability is disabled:
  1. Set the lower bit of the ISR Status field for the device.
  2. Send the appropriate PCI interrupt for the device.
• If MSI-X capability is enabled:
  1. Request the appropriate MSI-X interrupt message for the device, Queue Vector field sets the MSI-X Table entry number.
  2. If Queue Vector field value is NO_VECTOR, no interrupt message is requested for this event.

The driver interrupt handler should:

• If MSI-X capability is disabled: read the ISR Status field, which will reset it to zero. If the lower bit is zero, the interrupt was not for this device. Otherwise, the driver should look through the used rings of each virtqueue for the device, to see if any progress has been made by the device which requires servicing.
• If MSI-X capability is enabled: look through the used rings of each virtqueue mapped to the specific MSI-X vector for the device, to see if any progress has been made by the device which requires servicing.

4.1.3.4 Notification of Device Configuration Changes

Some virtio PCI devices can change the device configuration state, as reflected in the virtio header in the PCI configuration space. In this case:

• If MSI-X capability is disabled: an interrupt is delivered and the second lowest bit is set in the ISR Status field to indicate that the driver should re-examine the configuration space. Note that a single interrupt can indicate both that one or more virtqueue has been used and that the configuration space has changed: even if the config bit is set, virtqueues must be scanned.
• If MSI-X capability is enabled: an interrupt message is requested. The Configuration Vector field sets the MSI-X Table entry number to use. If Configuration Vector field value is NO_VECTOR, no interrupt message is requested for this event.

4.2 Virtio Over MMIO

Virtual environments without PCI support (a common situation in embedded devices models) might use simple memory mapped device ("virtio-mmio") instead of the PCI device.

The memory mapped virtio device behaviour is based on the PCI device specification. Therefore most of operations like device initialization, queues configuration and buffer transfers are nearly identical. Existing differences are described in the following sections.

4.2.1 MMIO Device Discovery

Unlike PCI, MMIO provides no generic device discovery. For systems using Flattened Device Trees the suggested format is:

4.2.2 MMIO Device Layout

MMIO virtio devices provides a set of memory mapped control registers, all 32 bits wide, followed by device-specific configuration space. The following table presents their names, offset from the base address, and whether they are read-only (R) or write-only (W) from the driver’s perspective:

MagicValue (0x000) - R

Magic value. Must be 0x74726976 (a Little Endian equivalent of a "virt" string).

Version (0x004) - R

Device version number. Devices compliant with this specification must return value 0x2.

DeviceID (0x008) - R

Virtio Subsystem Device ID. See 5 Device Types for possible values. Value zero (0x0) is invalid and devices returning this ID must be ignored by the guest.

VendorID (0x00c) - R

Virtio Subsystem Vendor ID.

DeviceFeatures (0x010) - R

Flags representing features the device supports. Reading from this register returns 32 consecutive flag bits, first bit depending on the last value written to the DeviceFeaturesSel register. Access to this register returns bits DeviceFeaturesSel*32 to (DeviceFeaturesSel*32)+31, eg. feature bits 0 to 31 if DeviceFeaturesSel is set to 0 and features bits 32 to 63 if DeviceFeaturesSel is set to 1. Also see 2.2 Feature Bits.

DeviceFeaturesSel (0x014) - W

Device (host) features word selection. Writing to this register selects a set of 32 device feature bits accessible by reading from the DeviceFeatures register. Device driver must write a value to the DeviceFeaturesSel register before reading from the DeviceFeatures register.

DriverFeatures (0x020) - W

Flags representing device features understood and activated by the driver. Writing to this register sets 32 consecutive flag bits, first bit depending on the last value written to the DriverFeaturesSel register. Access to this register sets bits DriverFeaturesSel*32 to (DriverFeaturesSel*32)+31, eg. feature bits 0 to 31 if DriverFeaturesSel is set to 0 and features bits 32 to 63 if DriverFeaturesSel is set to 1. Also see 2.2 Feature Bits.

DriverFeaturesSel (0x024) - W

Activated (guest) features word selection. Writing to this register selects a set of 32 activated feature bits accessible by writing to the DriverFeatures register. Device driver must write a value to the DriverFeaturesSel register before writing to the DriverFeatures register.

QueueSel (0x030) - W

Virtual queue index (first queue is 0). Writing to this register selects the virtual queue that the following operations on the QueueNumMax, QueueNum, QueueReady, QueueDescLow, QueueDescHigh, QueueAvailLow, QueueAvailHigh, QueueUsedLow and QueueUsedHigh registers apply to.

QueueNumMax (0x034) - R

Maximum virtual queue size. Reading from the register returns the maximum size of the queue the device is ready to process or zero (0x0) if the queue is not available. This applies to the queue selected by writing to QueueSel and is allowed only when QueueReady is set to zero (0x0), so when the queue is not in use.

QueueNum (0x038) - W

Virtual queue size. Queue size is the number of elements in the queue, therefore size of the Descriptor Table and both Available and Used rings. Writing to this register notifies the device what size of the queue the Driver will use. This applies to the queue selected by writing to QueueSel and is allowed only when QueueReady is set to zero (0x0), so when the queue is not in use.

QueueReady (0x03c) - RW

Virtual queue ready bit. Writing one (0x1) to this register notifies the device that the virtual queue is ready to be used. Reading from this register returns the last value written to it. Both read and write accesses apply to the queue selected by writing to QueueSel. When the Driver wants to stop using the queue it must write zero (0x0) to this register and read the value back to ensure synchronisation.

QueueNotify (0x050) - W

Queue notifier. Writing a queue index to this register notifies the device that there are new buffers to process in the queue.

InterruptStatus (0x60) - R

Interrupt status. Reading from this register returns a bit mask of interrupts asserted by the device. An interrupt is asserted if the corresponding bit is set, ie. equals one (1).

– Bit 0 | Used Ring Update This interrupt is asserted when the device has updated the Used Ring in at least one of the active virtual queues.

– Bit 1 | Configuration change This interrupt is asserted when configuration of the device has changed.

InterruptACK (0x064) - W

Interrupt acknowledge. Writing to this register notifies the device that the Driver finished handling interrupts. Set bits in the value clear the corresponding bits of the InterruptStatus register.

Status (0x070) - RW

Device status. Reading from this register returns the current device status flags. Writing non-zero values to this register sets the status flags, indicating the OS/driver progress. Writing zero (0x0) to this register triggers a device reset, including clearing all bits in the InterruptStatus register and ready bits in the QueueReady register for all queues in the device. See also p. 4.2.3.1 Device Initialization.

QueueDescLow (0x080) - W

QueueDescHigh (0x084) - W

Virtual queue’s Descriptor Table 64 bit long physical address. Writing to these two registers (lower 32 bits of the address to QueueDescLow, higher 32 bits to QueueDescHigh) notifies the device about location of the Descriptor Table of the queue selected by writing to the QueueSel register. It is allowed only when QueueReady is set to zero (0x0), so when the queue is not in use.

QueueAvailLow (0x090) - W

QueueAvailHigh (0x094) - W

Virtual queue’s Available Ring 64 bit long physical address. Writing to these two registers (lower 32 bits of the address to QueueAvailLow, higher 32 bits to QueueAvailHigh) notifies the device about location of the Available Ring of the queue selected by writing to the QueueSel register. It is allowed only when QueueReady is set to zero (0x0), so when the queue is not in use.

QueueUsedLow (0x0a0) - W

QueueUsedHigh (0x0a4) - W

Virtual queue’s Used Ring 64 bit long physical address. Writing to these two registers (lower 32 bits of the address to QueueUsedLow, higher 32 bits to QueueUsedHigh) notifies the device about location of the Used Ring of the queue selected by writing to the QueueSel register. It is allowed only when QueueReady is set to zero (0x0), so when the queue is not in use.

ConfigGeneration (0x0fc) - R

Configuration atomicity value. Changes every time the configuration noticeably changes. This means the device may only change the value after a configuration read operation, but it must change if there is any risk of a device seeing an inconsistent configuration state.

Config (0x100) - RW

Device-specific configuration space starts at an offset 0x100 and is accessed with byte alignment. Its meaning and size depends on the device and the driver.

All register values are organized as Little Endian.

Accessing memory locations not explicitly described above (or - in case of the configuration space - described in the device specification), writing to the registers described as "R" and reading from registers described as "W" is not permitted and can cause undefined behavior.

4.2.3 MMIO-specific Initialization And Device Operation

4.2.3.1 Device Initialization

The driver must start the device initialization by reading and checking values from the MagicValue and the Version registers. If both values are valid, it must read the DeviceID register and if its value is zero (0x0) must abort initialization and must not access any other register.

Further initialization must follow the procedure described in 3.1 Device Initialization.

4.2.3.2 Virtqueue Configuration

1. Select the queue writing its index (first queue is 0) to the QueueSel register.
2. Check if the queue is not already in use: read the QueueReady register, returned value should be zero (0x0).
3. Read maximum queue size (number of elements) from the QueueNumMax register. If the returned value is zero (0x0) the queue is not available.
4. Allocate and zero the queue pages, making sure the memory is physically contiguous. It is recommended to align the Used Ring to an optimal boundary (usually page size). Size of the allocated queue may be smaller than or equal to the maximum size returned by the device.
5. Notify the device about the queue size by writing the size to the QueueNum register.
6. Write physical addresses of the queue’s Descriptor Table, Available Ring and Used Ring to (respectively) the QueueDescLow/ QueueDescHigh, QueueAvailLow/QueueAvailHigh and QueueUsedLow/ QueueUsedHigh register pairs.
7. Write 0x1 to the QueueReady register.

4.2.3.3 Notifying The Device

The device is notified about new buffers available in a queue by writing the queue index to the QueueNum register.

4.2.3.4 Notifications From The Device

The memory mapped virtio device is using single, dedicated interrupt signal, which is raised when at least one of the interrupts described in the InterruptStatus register description is asserted. After receiving an interrupt, the driver must read the InterruptStatus register to check what caused the interrupt (see the register description). After the interrupt is handled, the driver must acknowledge it by writing a bit mask corresponding to the serviced interrupt to the InterruptACK register.

As documented in the InterruptStatus register description, the device may notify the driver about a new used buffer being available in the queue or about a change in the device configuration.

4.2.4 Legacy interface

The legacy MMIO transport used page-based addressing, resulting in a slightly different control register layout, the device initialization and the virtual queue configuration procedure.

The following list presents control registers layout, omitting descriptions of registers which did not change their function nor behaviour:

* Offset from the device base address | Direction | Name Description

* 0x000 | R | MagicValue

* 0x004 | R | Version Device version number. Legacy devices must return value 0x1.

* 0x008 | R | DeviceID

* 0x00c | R | VendorID

* 0x010 | R | HostFeatures

* 0x014 | W | HostFeaturesSel

* 0x020 | W | GuestFeatures

* 0x024 | W | GuestFeaturesSel

* 0x028 | W | GuestPageSize Guest page size. The driver must write the guest page size in bytes to the register during initialization, before any queues are used. This value must be a power of 2 and is used by the device to calculate the Guest address of the first queue page (see QueuePFN).

* 0x030 | W | QueueSel Virtual queue index (first queue is 0). Writing to this register selects the virtual queue that the following operations on the QueueNumMAx, QueueNum, QueueAlign and QueuePFN registers apply to.

* 0x034 | R | QueueNumMax Maximum virtual queue size. Reading from the register returns the maximum size of the queue the device is ready to process or zero (0x0) if the queue is not available. This applies to the queue selected by writing to the QueueSel and is allowed only when the QueuePFN is set to zero (0x0), so when the queue is not actively used.

* 0x038 | W | QueueNum Virtual queue size. Queue size is the number of elements in the queue, therefore size of the descriptor table and both available and used rings. Writing to this register notifies the device what size of the queue the Driver will use. This applies to the queue selected by writing to the QueueSel register.

* 0x03c | W | QueueAlign Used Ring alignment in the virtual queue. Writing to this register notifies the device about alignment boundary of the Used Ring in bytes. This value must be a power of 2 and applies to the queue selected by writing to the QueueSel register.

* 0x040 | RW | QueuePFN Guest physical page number of the virtual queue. Writing to this register notifies the device about location of the virtual queue in the Guest’s physical address space. This value is the index number of a page starting with the queue Descriptor Table. Value zero (0x0) means physical address zero (0x00000000) and is illegal. When the Driver stops using the queue it must write zero (0x0) to this register. Reading from this register returns the currently used page number of the queue, therefore a value other than zero (0x0) means that the queue is in use. Both read and write accesses apply to the queue selected by writing to the QueueSel register.

* 0x050 | W | QueueNotify

* 0x060 | R | InterruptStatus

* 0x064 | W | InterruptACK

* 0x070 | RW | Status Device status. Reading from this register returns the current device status flags. Writing non-zero values to this register sets the status flags, indicating the OS/driver progress. Writing zero (0x0) to this register triggers a device reset. This should include setting QueuePFN to zero (0x0) for all queues in the device. Also see 3.1 Device Initialization.

* 0x100+ | RW | Config

The virtual queue page size is defined by writing to the GuestPageSize register, as written by the guest. This must be done before the virtual queues are configured.

The virtual queue layout follows p. 2.4.1 Legacy Interfaces: A Note on Virtqueue Layout, with the alignment defined in the QueueAlign register.

The virtual queue is configured as follows:

1. Select the queue writing its index (first queue is 0) to the QueueSel register.
2. Check if the queue is not already in use: read the QueuePFN register, returned value should be zero (0x0).
3. Read maximum queue size (number of elements) from the QueueNumMax register. If the returned value is zero (0x0) the queue is not available.
4. Allocate and zero the queue pages in contiguous virtual memory, aligning the Used Ring to an optimal boundary (usually page size). Size of the allocated queue may be smaller than or equal to the maximum size returned by the device.
5. Notify the device about the queue size by writing the size to the QueueNum register.
6. Notify the device about the used alignment by writing its value in bytes to the QueueAlign register.
7. Write the physical number of the first page of the queue to the QueuePFN register.

Notification mechanisms did not change.

4.3 Virtio Over Channel I/O

S/390 based virtual machines support neither PCI nor MMIO, so a different transport is needed there.

virtio-ccw uses the standard channel I/O based mechanism used for the majority of devices on S/390. A virtual channel device with a special control unit type acts as proxy to the virtio device (similar to the way virtio-pci uses a PCI device) and configuration and operation of the virtio device is accomplished (mostly) via channel commands. This means virtio devices are discoverable via standard operating system algorithms, and adding virtio support is mainly a question of supporting a new control unit type.

As the S/390 is a big endian machine, the data structures transmitted via channel commands are big-endian: this is made clear by use of the types be16, be32 and be64.

4.3.1 Basic Concepts

As a proxy device, virtio-ccw uses a channel-attached I/O control unit with a special control unit type (0x3832) and a control unit model corresponding to the attached virtio device’s subsystem device ID, accessed via a virtual I/O subchannel and a virtual channel path of type 0x32. This proxy device is discoverable via normal channel subsystem device discovery (usually a STORE SUBCHANNEL loop) and answers to the basic channel commands, most importantly SENSE ID.

For a virtio-ccw proxy device, SENSE ID will return the following information:

A driver for virtio-ccw devices MUST check for a control unit type of 0x3832 and MUST ignore the device type and model.

In addition to the basic channel commands, virtio-ccw defines a set of channel commands related to configuration and operation of virtio:

The virtio-ccw device acts like a normal channel device, as specified in [S390 PoP] and [S390 Common I/O]. In particular:

• A device must post a unit check with command reject for any command it does not support.
• If a driver did not suppress length checks for a channel command, the device must present a subchannel status as detailed in the architecture when the actual length did not match the expected length.
• If a driver did suppress length checks for a channel command, the device must present a check condition if the transmitted data does not contain enough data to process the command. If the driver submitted a buffer that was too long, the device should accept the command. The driver should attempt to provide the correct length even if it suppresses length checks.

4.3.2 Device Initialization

virtio-ccw uses several channel commands to set up a device.

4.3.2.1 Setting the Virtio Revision

CCW_CMD_SET_VIRTIO_REV is issued by the driver to set the revision of the virtio-ccw transport it intends to drive the device with. It uses the following communication structure:

revision contains the desired revision id, length the length of the data portion and data revision-dependent additional desired options.

The following values are supported:

Note that a change in the virtio standard does not neccessarily correspond to a change in the virtio-ccw revision.

A device must post a unit check with command reject for any revision it does not support. For any invalid combination of revision, length and data, it must post a unit check with command reject as well. A non-transitional device must reject revision id 0.

A driver should start with trying to set the highest revision it supports and continue with lower revisions if it gets a command reject.

A driver must not issue any other virtio-ccw specific channel commands prior to setting the revision.

A device must answer with command reject to any virtio-ccw specific channel command that is not contained in the revision selected by the driver.

After a revision has been successfully selected by the driver, it must not attempt to select a different revision. A device must answer to any such attempt with a command reject.

A device must treat the revision as unset from the time the associated subchannel has been enabled until a revision has been successfully set by the driver. This implies that revisions are not persistent across disabling and enabling of the associated subchannel.

4.3.2.1.1 Legacy Interfaces: A Note on Setting the Virtio Revision

A legacy driver will not issue the CCW_CMD_SET_VIRTIO_REV prior to issueing other virtio-ccw specific channel commands. A non-transitional device therefore must answer any such attempts with a command reject. A transitional device must assume in this case that the driver is a legacy driver and continue as if the driver selected revision 0. This implies that the device must reject any command not valid for revision 0, including a subsequent CCW_CMD_SET_VIRTIO_REV.

4.3.2.2 Configuring a Virtqueue

CCW_CMD_READ_VQ_CONF is issued by the driver to obtain information about a queue. It uses the following structure for communicating:

The requested number of buffers for queue index is returned in max_num.

Afterwards, CCW_CMD_SET_VQ is issued by the driver to inform the device about the location used for its queue. The transmitted structure is

desc, avail and used contain the guest addresses for the descriptor table, available ring and used ring for queue index, respectively. The actual virtqueue size (number of allocated buffers) is transmitted in num. res0 is reserved and must be ignored by the device.

4.3.2.2.1 Legacy Interface: A Note on Configuring a Virtqueue

queue contains the guest address for queue index, num the number of buffers and align the alignment.

4.3.2.3 Virtqueue Layout

The virtqueue is physically contiguous, with padded added to make the used ring meet the align value:

The calculation for total size is as follows:

4.3.2.4 Communicating Status Information

The driver can change the status of a device via the CCW_CMD_WRITE_STATUS command, which transmits an 8 bit status value.

4.3.2.5 Handling Device Features

Feature bits are arranged in an array of 32 bit values, making for a total of 8192 feature bits. Feature bits are in little-endian byte order.

The CCW commands dealing with features use the following communication block:

features are the 32 bits of features currently accessed, while index describes which of the feature bit values is to be accessed.

The guest may obtain the device’s device feature set via the CCW_CMD_READ_FEAT command. The device stores the features at index to features.

For communicating its supported features to the device, the driver may use the CCW_CMD_WRITE_FEAT command, denoting a features/index combination.

4.3.2.6 Device Configuration

The device’s configuration space is located in host memory. It is the same size as the standard PCI configuration space.

To obtain information from the configuration space, the driver may use CCW_CMD_READ_CONF, specifying the guest memory for the device to write to.

For changing configuration information, the driver may use CCW_CMD_WRITE_CONF, specifying the guest memory for the device to read from.

In both cases, the complete configuration space is transmitted. This allows the driver to compare the new configuration space with the old version, and keep a generation count internally whenever it changes.

4.3.2.7 Setting Up Indicators

In order to set up the indicator bits for host->guest notification, the driver uses different channel commands depending on whether it wishes to use traditional I/O interrupts tied to a subchannel or adapter I/O interrupts for virtqueue notifications. For any given device, the two mechanisms are mutually exclusive.

For the configuration change indicators, only a mechanism using traditional I/O interrupts is provided, regardless of whether traditional or adapter I/O interrupts are used for virtqueue notifications.

4.3.2.7.1 Setting Up Classic Queue Indicators

To communicate the location of the indicator bits for host->guest notification, the driver uses the CCW_CMD_SET_IND command, pointing to a location containing the guest address of the indicators in a 64 bit value.

If the driver has already set up two-staged queue indicators via the CCW_CMD_SET_IND_ADAPTER command, the device MUST post a unit check with command reject to any subsequent CCW_CMD_SET_IND command.

4.3.2.7.2 Setting Up Configuration Change Indicators

To communicate the location of the indicator bits used in the configuration change host->guest notification, the driver issues the CCW_CMD_SET_CONF_IND command, pointing to a location containing the guest address of the indicators in a 64 bit value.

4.3.2.7.3 Setting Up Two-Stage Queue Indicators

• a summary indicator byte covering the virtqueues for one or more virtio-ccw proxy devices
• a set of contigous indicator bits for the virtqueues for a virtio-ccw proxy device

To communicate the location of the summary and queue indicator bits, the driver uses the CCW_CMD_SET_IND_ADAPTER command with the following payload:

summary_indicator contains the guest address of the 8 bit summary indicator. indicator contains the guest address of an area wherin the indicators for the devices are contained, starting at bit_nr, one bit per virtqueue of the device. Bit numbers start at the left. isc contains the I/O interruption subclass to be used for the adapter I/O interrupt. It may be different from the isc used by the proxy virtio-ccw device’s subchannel.

If the driver has already set up classic queue indicators via the CCW_CMD_SET_IND command, the device MUST post a unit check with command reject to any subsequent CCW_CMD_SET_IND_ADAPTER command.

4.3.2.7.4 Legacy Interfaces: A Note on Setting Up Indicators

4.3.3 Device Operation

4.3.3.1 Host->Guest Notification

There are two modes of operation regarding host->guest notifcation, classic I/O interrupts and adapter I/O interrupts. The mode to be used is determined by the driver by using CCW_CMD_SET_IND respectively CCW_CMD_SET_IND_ADAPTER to set up queue indicators.

For configuration changes, the driver will always use classic I/O interrupts.

4.3.3.1.1 Notification via Classic I/O Interrupts

For notifying the driver of virtqueue buffers, the device sets the corresponding bit in the guest-provided indicators. If an interrupt is not already pending for the subchannel, the device generates an unsolicited I/O interrupt.

If the device wants to notify the driver about configuration changes, it sets bit 0 in the configuration indicators and generates an unsolicited I/O interrupt, if needed. This also applies if adapter I/O interrupts are used for queue notifications.

4.3.3.1.2 Notification via Adapter I/O Interrupts

For notifying the driver of virtqueue buffers, the device sets the bit in the guest-provided indicator area at the corresponding offset. The guest-provided summary indicator is also set. An adapter I/O interrupt for the corresponding interruption subclass is generated. The device SHOULD only generate an adapter I/O interrupt if the summary indicator had not been set prior to notification. The driver MUST clear the summary indicator after receiving an adapter I/O interrupt before it processes the queue indicators.

4.3.3.1.3 Legacy Interfaces: A Note on Host->Guest Notification

4.3.3.2 Guest->Host Notification

For notifying the device of virtqueue buffers, the driver unfortunately can’t use a channel command (the asynchronous characteristics of channel I/O interact badly with the host block I/O backend). Instead, it uses a diagnose 0x500 call with subcode 3 specifying the queue, as follows:

Host cookie is an optional per-virtqueue 64 bit value that can be used by the hypervisor to speed up the notification execution. For each notification, the output value is returned in GPR2 and should be passed in GPR4 for the next notification:

4.3.3.3 Early printk for Virtio Consoles

For the early printk mechanism, diagnose 0x500 with subcode 0 is used.

4.3.3.4 Resetting Devices

In order to reset a device, a driver may send the CCW_CMD_VDEV_RESET command.

---

5 Device Types

The following device IDs are used to identify different types of virtio devices. Some device IDs are reserved for devices which are not currently defined in this standard.

Discovering what devices are available and their type is bus-dependent.

5.1 Network Device

The virtio network device is a virtual ethernet card, and is the most complex of the devices supported so far by virtio. It has enhanced rapidly and demonstrates clearly how support for new features should be added to an existing device. Empty buffers are placed in one virtqueue for receiving packets, and outgoing packets are enqueued into another for transmission in that order. A third command queue is used to control advanced filtering features.

5.1.1 Device ID

1

5.1.2 Virtqueues

0

receiveq

1

transmitq

2

controlq

Virtqueue 2 only exists if VIRTIO_NET_F_CTRL_VQ set.

5.1.3 Feature bits

VIRTIO_NET_F_CSUM (0)

Device handles packets with partial checksum

VIRTIO_NET_F_GUEST_CSUM (1)

Driver handles packets with partial checksum

VIRTIO_NET_F_CTRL_GUEST_OFFLOADS (2)

Control channel offloads reconfiguration support.

VIRTIO_NET_F_MAC (5)

Device has given MAC address.

VIRTIO_NET_F_GUEST_TSO4 (7)

Driver can receive TSOv4.

VIRTIO_NET_F_GUEST_TSO6 (8)

Driver can receive TSOv6.

VIRTIO_NET_F_GUEST_ECN (9)

Driver can receive TSO with ECN.

VIRTIO_NET_F_GUEST_UFO (10)

Driver can receive UFO.

VIRTIO_NET_F_HOST_TSO4 (11)

Device can receive TSOv4.

VIRTIO_NET_F_HOST_TSO6 (12)

Device can receive TSOv6.

VIRTIO_NET_F_HOST_ECN (13)

Device can receive TSO with ECN.

VIRTIO_NET_F_HOST_UFO (14)

Device can receive UFO.

VIRTIO_NET_F_MRG_RXBUF (15)

Driver can merge receive buffers.

VIRTIO_NET_F_STATUS (16)

Configuration status field is available.

VIRTIO_NET_F_CTRL_VQ (17)

Control channel is available.

VIRTIO_NET_F_CTRL_RX (18)

Control channel RX mode support.

VIRTIO_NET_F_CTRL_VLAN (19)

Control channel VLAN filtering.

VIRTIO_NET_F_GUEST_ANNOUNCE(21)

Driver can send gratuitous packets.

5.1.3.1 Legacy Interface: Feature bits

VIRTIO_NET_F_GSO (6)

Device handles packets with any GSO type.

This was supposed to indicate segmentation offload support, but upon further investigation it became clear that multiple bits were required.

5.1.4 Device configuration layout

Two configuration fields are currently defined. The mac address field always exists (though is only valid if VIRTIO_NET_F_MAC is set), and the status field only exists if VIRTIO_NET_F_STATUS is set. Two read-only bits are currently defined for the status field: VIRTIO_NET_S_LINK_UP and VIRTIO_NET_S_ANNOUNCE.

5.1.4.1 Legacy Interface: Device configuration layout

For legacy devices, the status field in struct virtio_net_config is the native endian of the guest rather than (necessarily) little-endian.

5.1.5 Device Initialization

1. The initialization routine should identify the receive and transmission virtqueues.
2. If the VIRTIO_NET_F_MAC feature bit is set, the configuration space “mac” entry indicates the “physical” address of the the network card, otherwise a private MAC address should be assigned. All drivers are expected to negotiate this feature if it is set.
3. If the VIRTIO_NET_F_CTRL_VQ feature bit is negotiated, identify the control virtqueue.
4. If the VIRTIO_NET_F_STATUS feature bit is negotiated, the link status can be read from the bottom bit of the “status” config field. Otherwise, the link should be assumed active.
5. The receive virtqueue should be filled with receive buffers. This is described in detail below in “Setting Up Receive Buffers”.
6. A driver can indicate that it will generate checksumless packets by negotating the VIRTIO_NET_F_CSUM feature. This “checksum offload” is a common feature on modern network cards.
7. If that feature is negotiated⁵, a driver can use TCP or UDP segmentation offload by negotiating the VIRTIO_NET_F_HOST_TSO4 (IPv4 TCP), VIRTIO_NET_F_HOST_TSO6 (IPv6 TCP) and VIRTIO_NET_F_HOST_UFO (UDP fragmentation) features. It should not send TCP packets requiring segmentation offload which have the Explicit Congestion Notification bit set, unless the VIRTIO_NET_F_HOST_ECN feature is negotiated.⁶
8. The converse features are also available: a driver can save the virtual device some work by negotiating these features.⁷ The VIRTIO_NET_F_GUEST_CSUM feature indicates that partially checksummed packets can be received, and if it can do that then the VIRTIO_NET_F_GUEST_TSO4, VIRTIO_NET_F_GUEST_TSO6, VIRTIO_NET_F_GUEST_UFO and VIRTIO_NET_F_GUEST_ECN are the input equivalents of the features described above. See 5.1.6.3 Setting Up Receive Buffers and 5.1.6.3 Setting Up Receive Buffers below.

5.1.6 Device Operation

Packets are transmitted by placing them in the transmitq, and buffers for incoming packets are placed in the receiveq. In each case, the packet itself is preceeded by a header:

The controlq is used to control device features such as filtering.

5.1.6.1 Legacy Interface: Device Operation

For legacy devices, the fields in struct virtio_net_hdr are the native endian of the guest rather than (necessarily) little-endian.

5.1.6.2 Packet Transmission

Transmitting a single packet is simple, but varies depending on the different features the driver negotiated.

1. If the driver negotiated VIRTIO_NET_F_CSUM, and the packet has not been fully checksummed, then the virtio_net_hdr’s fields are set as follows. Otherwise, the packet must be fully checksummed, and flags is zero.
   • flags has the VIRTIO_NET_HDR_F_NEEDS_CSUM set,
   • csum_start is set to the offset within the packet to begin checksumming, and
   • csum_offset indicates how many bytes after the csum_start the new (16 bit ones’ complement) checksum should be placed.

   For example, consider a partially checksummed TCP (IPv4) packet. It will have a 14 byte ethernet header and 20 byte IP header followed by the TCP header (with the TCP checksum field 16 bytes into that header). csum_start will be 14+20 = 34 (the TCP checksum includes the header), and csum_offset will be 16. The value in the TCP checksum field should be initialized to the sum of the TCP pseudo header, so that replacing it by the ones’ complement checksum of the TCP header and body will give the correct result.

2. If the driver negotiated VIRTIO_NET_F_HOST_TSO4, TSO6 or UFO, and the packet requires TCP segmentation or UDP fragmentation, then the “gso_type” field is set to VIRTIO_NET_HDR_GSO_TCPV4, TCPV6 or UDP. (Otherwise, it is set to VIRTIO_NET_HDR_GSO_NONE). In this case, packets larger than 1514 bytes can be transmitted: the metadata indicates how to replicate the packet header to cut it into smaller packets. The other gso fields are set:
   • hdr_len is a hint to the device as to how much of the header needs to be kept to copy into each packet, usually set to the length of the headers, including the transport header.⁸
   • gso_size is the maximum size of each packet beyond that header (ie. MSS).
   • If the driver negotiated the VIRTIO_NET_F_HOST_ECN feature, the VIRTIO_NET_HDR_GSO_ECN bit may be set in “gso_type” as well, indicating that the TCP packet has the ECN bit set.⁹
3. If the driver negotiated the VIRTIO_NET_F_MRG_RXBUF feature, the num_buffers field is set to zero.
4. The header and packet are added as one output buffer to the transmitq, and the device is notified of the new entry (see 5.1.5 Device Initialization).¹⁰

5.1.6.2.1 Packet Transmission Interrupt

The normal behavior in this interrupt handler is to retrieve and new descriptors from the used ring and free the corresponding headers and packets.

5.1.6.3 Setting Up Receive Buffers

It is generally a good idea to keep the receive virtqueue as fully populated as possible: if it runs out, network performance will suffer.

If the VIRTIO_NET_F_GUEST_TSO4, VIRTIO_NET_F_GUEST_TSO6 or VIRTIO_NET_F_GUEST_UFO features are used, the Driver will need to accept packets of up to 65550 bytes long (the maximum size of a TCP or UDP packet, plus the 14 byte ethernet header), otherwise 1514. bytes. So unless VIRTIO_NET_F_MRG_RXBUF is negotiated, every buffer in the receive queue needs to be at least this length.¹¹

If VIRTIO_NET_F_MRG_RXBUF is negotiated, each buffer must be at least the size of the struct virtio_net_hdr.

5.1.6.3.1 Packet Receive Interrupt

Processing packet involves:

1. If the driver negotiated the VIRTIO_NET_F_MRG_RXBUF feature, then the “num_buffers” field indicates how many descriptors this packet is spread over (including this one). This allows receipt of large packets without having to allocate large buffers. In this case, there will be at least “num_buffers” in the used ring, and they should be chained together to form a single packet. The other buffers will not begin with a struct virtio_net_hdr.
2. If the VIRTIO_NET_F_MRG_RXBUF feature was not negotiated, or the “num_buffers” field is one, then the entire packet will be contained within this buffer, immediately following the struct virtio_net_hdr.
3. If the VIRTIO_NET_F_GUEST_CSUM feature was negotiated, the VIRTIO_NET_HDR_F_NEEDS_CSUM bit in the “flags” field may be set: if so, the checksum on the packet is incomplete and the “ csum_start” and “csum_offset” fields indicate how to calculate it (see Packet Transmission point 1).
4. If the VIRTIO_NET_F_GUEST_TSO4, TSO6 or UFO options were negotiated, then the “gso_type” may be something other than VIRTIO_NET_HDR_GSO_NONE, and the “gso_size” field indicates the desired MSS (see Packet Transmission point 2).

5.1.6.4 Control Virtqueue

The driver uses the control virtqueue (if VIRTIO_NET_F_VTRL_VQ is negotiated) to send commands to manipulate various features of the device which would not easily map into the configuration space.

All commands are of the following form:

The class, command and command-specific-data are set by the driver, and the device sets the ack byte. There is little it can do except issue a diagnostic if the ack byte is not VIRTIO_NET_OK.

5.1.6.4.1 Packet Receive Filtering

Note that in general, these commands are best-effort: unwanted packets may still arrive.

5.1.6.4.2 Setting Promiscuous Mode

The class VIRTIO_NET_CTRL_RX has two commands: VIRTIO_NET_CTRL_RX_PROMISC turns promiscuous mode on and off, and VIRTIO_NET_CTRL_RX_ALLMULTI turns all-multicast receive on and off. The command-specific-data is one byte containing 0 (off) or 1 (on).

5.1.6.4.3 Setting MAC Address Filtering

The device can filter incoming packets by any number of destination MAC addresses.¹² This table is set using the class VIRTIO_NET_CTRL_MAC and the command VIRTIO_NET_CTRL_MAC_TABLE_SET. The command-specific-data is two variable length tables of 6-byte MAC addresses. The first table contains unicast addresses, and the second contains multicast addresses.

5.1.6.4.3.1 Legacy Interface: Setting MAC Address Filtering

5.1.6.4.4 VLAN Filtering

Both the VIRTIO_NET_CTRL_VLAN_ADD and VIRTIO_NET_CTRL_VLAN_DEL command take a little-endian 16-bit VLAN id as the command-specific-data.

5.1.6.4.4.1 Legacy Interface: VLAN Filtering

5.1.6.4.5 Gratuitous Packet Sending

The Driver needs to check VIRTIO_NET_S_ANNOUNCE bit in status field when it notices the changes of device configuration. The command VIRTIO_NET_CTRL_ANNOUNCE_ACK is used to indicate that driver has recevied the notification and device would clear the VIRTIO_NET_S_ANNOUNCE bit in the status filed after it received this command.

Processing this notification involves:

1. Sending the gratuitous packets or marking there are pending gratuitous packets to be sent and letting deferred routine to send them.
2. Sending VIRTIO_NET_CTRL_ANNOUNCE_ACK command through control vq.

5.1.6.4.6 Offloads State Configuration

5.1.6.4.6.1 Setting Offloads State

The class VIRTIO_NET_CTRL_GUEST_OFFLOADS has one command: VIRTIO_NET_CTRL_GUEST_OFFLOADS_SET applies the new offloads configuration.

le64 value passed as command data is a bitmask, bits set define offloads to be enabled, bits cleared - offloads to be disabled.

There is a corresponding device feature for each offload. Upon feature negotiation corresponding offload gets enabled to preserve backward compartibility.

Corresponding feature must be negotiated at startup in order to allow dynamic change of specific offload state.

5.1.6.4.6.2 Legacy Interface: Setting Offloads State

5.2 Block Device

The virtio block device is a simple virtual block device (ie. disk). Read and write requests (and other exotic requests) are placed in the queue, and serviced (probably out of order) by the device except where noted.

5.2.1 Device ID

2

5.2.2 Virtqueues

0

requestq

5.2.3 Feature bits

VIRTIO_BLK_F_SIZE_MAX (1)

Maximum size of any single segment is in “size_max”.

VIRTIO_BLK_F_SEG_MAX (2)

Maximum number of segments in a request is in “seg_max”.

VIRTIO_BLK_F_GEOMETRY (4)

Disk-style geometry specified in “ geometry”.

VIRTIO_BLK_F_RO (5)

Device is read-only.

VIRTIO_BLK_F_BLK_SIZE (6)

Block size of disk is in “blk_size”.

VIRTIO_BLK_F_TOPOLOGY (10)

Device exports information on optimal I/O alignment.

5.2.3.1 Legacy Interface: Feature bits

VIRTIO_BLK_F_BARRIER (0)

Device supports request barriers.

VIRTIO_BLK_F_SCSI (7)

Device supports scsi packet commands.

VIRTIO_BLK_F_FLUSH (9)

Cache flush command support.

VIRTIO_BLK_F_CONFIG_WCE (11)

Device can toggle its cache between writeback and writethrough modes.

VIRTIO_BLK_F_FLUSH was also called VIRTIO_BLK_F_WCE: Legacy drivers should only negotiate this feature if they are capable of sending VIRTIO_BLK_T_FLUSH commands.

5.2.3.2 Device configuration layout

The capacity of the device (expressed in 512-byte sectors) is always present. The availability of the others all depend on various feature bits as indicated above.

5.2.3.2.1 Legacy Interface: Device configuration layout

5.2.4 Device Initialization

1. The device size should be read from the “capacity” configuration field. No requests should be submitted which goes beyond this limit.
2. If the VIRTIO_BLK_F_BLK_SIZE feature is negotiated, the blk_size field can be read to determine the optimal sector size for the driver to use. This does not affect the units used in the protocol (always 512 bytes), but awareness of the correct value can affect performance.
3. If the VIRTIO_BLK_F_RO feature is set by the device, any write requests will fail.
4. If the VIRTIO_BLK_F_TOPOLOGY feature is negotiated, the fields in the topology struct can be read to determine the physical block size and optimal I/O lengths for the driver to use. This also does not affect the units in the protocol, only performance.

5.2.4.1 Legacy Interface: Device Initialization

The reserved field used to be called writeback. If the VIRTIO_BLK_F_CONFIG_WCE feature is offered, the cache mode should be read from the writeback field of the configuration if available; the driver can also write to the field in order to toggle the cache between writethrough (0) and writeback (1) mode. If the feature is not available, the driver can instead look at the result of negotiating VIRTIO_BLK_F_FLUSH: the cache will be in writeback mode after reset if and only if VIRTIO_BLK_F_FLUSH is negotiated.

Some older legacy devices did not operate in writethrough mode even after a driver announced lack of support for VIRTIO_BLK_F_FLUSH.

5.2.5 Device Operation

The driver queues requests to the virtqueue, and they are used by the device (not necessarily in order). Each request is of form:

The type of the request is either a read (VIRTIO_BLK_T_IN), a write (VIRTIO_BLK_T_OUT), or a flush (VIRTIO_BLK_T_FLUSH or VIRTIO_BLK_T_FLUSH_OUT¹³ ).

The sector number indicates the offset (multiplied by 512) where the read or write is to occur. This field is unused and set to 0 for scsi packet commands and for flush commands.

The final status byte is written by the device: either VIRTIO_BLK_S_OK for success, VIRTIO_BLK_S_IOERR for device or driver error or VIRTIO_BLK_S_UNSUPP for a request unsupported by device:

Any writes completed before the submission of the flush command should be committed to non-volatile storage by the device.

5.2.5.1 Legacy Interface: Device Operation

For legacy devices, the fields in struct virtio_blk_req are the native endian of the guest rather than (necessarily) little-endian.

The ’reserved’ field was previously called ioprio. The ioprio field is a hint about the relative priorities of requests to the device: higher numbers indicate more important requests.

If the device has VIRTIO_BLK_F_BARRIER feature the high bit (VIRTIO_BLK_T_BARRIER) indicates that this request acts as a barrier and that all preceeding requests must be complete before this one, and all following requests must not be started until this is complete. Note that a barrier does not flush caches in the underlying backend device in host, and thus does not serve as data consistency guarantee. Driver must use FLUSH request to flush the host cache.

If the device has VIRTIO_BLK_F_SCSI feature, it can also support scsi packet command requests, each of these requests is of form:

A request type can also be a scsi packet command (VIRTIO_BLK_T_SCSI_CMD or VIRTIO_BLK_T_SCSI_CMD_OUT). The two types are equivalent, the device does not distinguish between them:

The cmd field is only present for scsi packet command requests, and indicates the command to perform. This field must reside in a single, separate read-only buffer; command length can be derived from the length of this buffer.

Note that these first three (four for scsi packet commands) fields are always read-only: the data field is either read-only or write-only, depending on the request. The size of the read or write can be derived from the total size of the request buffers.

The sense field is only present for scsi packet command requests, and indicates the buffer for scsi sense data.

The data_len field is only present for scsi packet command requests, this field is deprecated, and should be ignored by the driver. Historically, devices copied data length there.

The sense_len field is only present for scsi packet command requests and indicates the number of bytes actually written to the sense buffer.

The residual field is only present for scsi packet command requests and indicates the residual size, calculated as data length - number of bytes actually transferred.

Historically, devices assumed that the fields type, ioprio and sector reside in a single, separate read-only buffer; the fields errors, data_len, sense_len and residual reside in a single, separate write-only buffer; the sense field in a separate write-only buffer of size 96 bytes, by itself; the fields errors, data_len, sense_len and residual in a single write-only buffer; and the status field is a separate read-only buffer of size 1 byte, by itself.

5.3 Console Device

The virtio console device is a simple device for data input and output. A device may have one or more ports. Each port has a pair of input and output virtqueues. Moreover, a device has a pair of control IO virtqueues. The control virtqueues are used to communicate information between the device and the driver about ports being opened and closed on either side of the connection, indication from the device about whether a particular port is a console port, adding new ports, port hot-plug/unplug, etc., and indication from the driver about whether a port or a device was successfully added, port open/close, etc.. For data IO, one or more empty buffers are placed in the receive queue for incoming data and outgoing characters are placed in the transmit queue.

5.3.1 Device ID

3

5.3.2 Virtqueues

0

receiveq(port0)

1

transmitq(port0)

2

control receiveq

3

control transmitq

4

receiveq(port1)

5

transmitq(port1)

…

Ports 2 onwards only exist if VIRTIO_CONSOLE_F_MULTIPORT is set.

5.3.3 Feature bits

VIRTIO_CONSOLE_F_SIZE (0) Configuration cols and rows fields are valid.

VIRTIO_CONSOLE_F_MULTIPORT(1) Device has support for multiple ports; configuration fields nr_ports and max_nr_ports are valid and control virtqueues will be used.

5.3.4 Device configuration layout

The size of the console is supplied in the configuration space if the VIRTIO_CONSOLE_F_SIZE feature is set. Furthermore, if the VIRTIO_CONSOLE_F_MULTIPORT feature is set, the maximum number of ports supported by the device can be fetched.

5.3.4.1 Legacy Interface: Device configuration layout

For legacy devices, the fields in struct virtio_console_config are the native endian of the guest rather than (necessarily) little-endian.

5.3.5 Device Initialization

1. If the VIRTIO_CONSOLE_F_SIZE feature is negotiated, the driver can read the console dimensions from the configuration fields.
2. If the VIRTIO_CONSOLE_F_MULTIPORT feature is negotiated, the driver can spawn multiple ports, not all of which may be attached to a console. Some could be generic ports. In this case, the control virtqueues are enabled and according to the max_nr_ports configuration-space value, the appropriate number of virtqueues are created. A control message indicating the driver is ready is sent to the device. The device can then send control messages for adding new ports to the device. After creating and initializing each port, a VIRTIO_CONSOLE_PORT_READY control message is sent to the device for that port so the device can let us know of any additional configuration options set for that port.
3. The receiveq for each port is populated with one or more receive buffers.

5.3.6 Device Operation

1. For output, a buffer containing the characters is placed in the port’s transmitq.¹⁴
2. When a buffer is used in the receiveq (signalled by an interrupt), the contents is the input to the port associated with the virtqueue for which the notification was received.
3. If the driver negotiated the VIRTIO_CONSOLE_F_SIZE feature, a configuration change interrupt may occur. The updated size can be read from the configuration fields.
4. If the driver negotiated the VIRTIO_CONSOLE_F_MULTIPORT feature, active ports are announced by the device using the VIRTIO_CONSOLE_PORT_ADD control message. The same message is used for port hot-plug as well.
5. If the device specified a port ‘name’, a sysfs attribute is created with the name filled in, so that udev rules can be written that can create a symlink from the port’s name to the char device for port discovery by applications in the driver.
6. Changes to ports’ state are effected by control messages. Appropriate action is taken on the port indicated in the control message. The layout of the structure of the control buffer and the events associated are:
     struct virtio_console_control { 
       le32 id;    /* Port number */ 
       le16 event; /* The kind of control event */ 
       le16 value; /* Extra information for the event */ 
     }; 
    
     /* Some events for the internal messages (control packets) */ 
     #define VIRTIO_CONSOLE_DEVICE_READY     0 
     #define VIRTIO_CONSOLE_PORT_ADD         1 
     #define VIRTIO_CONSOLE_PORT_REMOVE      2 
     #define VIRTIO_CONSOLE_PORT_READY       3 
     #define VIRTIO_CONSOLE_CONSOLE_PORT     4 
     #define VIRTIO_CONSOLE_RESIZE           5 
     #define VIRTIO_CONSOLE_PORT_OPEN        6 
     #define VIRTIO_CONSOLE_PORT_NAME        7

5.3.6.1 Legacy Interface: Device Operation

For legacy devices, the fields in struct virtio_console_control are the native endian of the guest rather than (necessarily) little-endian.

5.4 Entropy Device

The virtio entropy device supplies high-quality randomness for guest use.

5.4.1 Device ID

4

5.4.2 Virtqueues

0

requestq

5.4.3 Feature bits

None currently defined

5.4.4 Device configuration layout

None currently defined.

5.4.5 Device Initialization

1. The virtqueue is initialized

5.4.6 Device Operation

When the driver requires random bytes, it places the descriptor of one or more buffers in the queue. It will be completely filled by random data by the device.

5.5 Memory Balloon Device

The virtio memory balloon device is a primitive device for managing guest memory: the device asks for a certain amount of memory, and the driver supplies it (or withdraws it, if the device has more than it asks for). This allows the guest to adapt to changes in allowance of underlying physical memory. If the feature is negotiated, the device can also be used to communicate guest memory statistics to the host.

5.5.1 Device ID

5

5.5.2 Virtqueues

0

inflateq

1

deflateq

2

statsq.

Virtqueue 2 only exists if VIRTIO_BALLON_F_STATS_VQ set.

5.5.3 Feature bits

VIRTIO_BALLOON_F_MUST_TELL_HOST (0)

Host must be told before pages from the balloon are used.

VIRTIO_BALLOON_F_STATS_VQ (1)

A virtqueue for reporting guest memory statistics is present.

5.5.4 Device configuration layout

Both fields of this configuration are always available.

5.5.4.1 Legacy Interface: Device configuration layout

Note that these fields are always little endian, despite convention that legacy device fields are guest endian.

5.5.5 Device Initialization

1. The inflate and deflate virtqueues are identified.
2. If the VIRTIO_BALLOON_F_STATS_VQ feature bit is negotiated:
   1. Identify the stats virtqueue.
   2. Add one empty buffer to the stats virtqueue and notify the device.

Device operation begins immediately.

5.5.6 Device Operation

The device is driven by the receipt of a configuration change interrupt.

1. The “num_pages” configuration field is examined. If this is greater than the “actual” number of pages, memory must be given to the balloon. If it is less than the “actual” number of pages, memory may be taken back from the balloon for general use.
2. To supply memory to the balloon (aka. inflate):
   1. The driver constructs an array of addresses of unused memory pages. These addresses are divided by 4096¹⁵ and the descriptor describing the resulting 32-bit array is added to the inflateq.
3. To remove memory from the balloon (aka. deflate):
   1. The driver constructs an array of addresses of memory pages it has previously given to the balloon, as described above. This descriptor is added to the deflateq.
   2. If the VIRTIO_BALLOON_F_MUST_TELL_HOST feature is negotiated, the guest may not use these requested pages until that descriptor in the deflateq has been used by the device.
   3. Otherwise, the guest may begin to re-use pages previously given to the balloon before the device has acknowledged their withdrawl.¹⁶
4. In either case, once the device has completed the inflation or deflation, the “actual” field of the configuration should be updated to reflect the new number of pages in the balloon.¹⁷

5.5.6.1 Memory Statistics

The stats virtqueue is atypical because communication is driven by the device (not the driver). The channel becomes active at driver initialization time when the driver adds an empty buffer and notifies the device. A request for memory statistics proceeds as follows:

1. The device pushes the buffer onto the used ring and sends an interrupt.
2. The driver pops the used buffer and discards it.
3. The driver collects memory statistics and writes them into a new buffer.
4. The driver adds the buffer to the virtqueue and notifies the device.
5. The device pops the buffer (retaining it to initiate a subsequent request) and consumes the statistics.

Each statistic consists of a 16 bit tag and a 64 bit value. All statistics are optional and the driver may choose which ones to supply. To guarantee backwards compatibility, unsupported statistics should be omitted.

5.5.6.1.1 Legacy Interface: Memory Statistics

5.5.6.2 Memory Statistics Tags

VIRTIO_BALLOON_S_SWAP_IN (0)

The amount of memory that has been swapped in (in bytes).

VIRTIO_BALLOON_S_SWAP_OUT (1)

The amount of memory that has been swapped out to disk (in bytes).

VIRTIO_BALLOON_S_MAJFLT (2)

The number of major page faults that have occurred.

VIRTIO_BALLOON_S_MINFLT (3)

The number of minor page faults that have occurred.

VIRTIO_BALLOON_S_MEMFREE (4)

The amount of memory not being used for any purpose (in bytes).

VIRTIO_BALLOON_S_MEMTOT (5)

The total amount of memory available (in bytes).

5.6 SCSI Host Device

The virtio SCSI host device groups together one or more virtual logical units (such as disks), and allows communicating to them using the SCSI protocol. An instance of the device represents a SCSI host to which many targets and LUNs are attached.

The virtio SCSI device services two kinds of requests:

• command requests for a logical unit;
• task management functions related to a logical unit, target or command.

The device is also able to send out notifications about added and removed logical units. Together, these capabilities provide a SCSI transport protocol that uses virtqueues as the transfer medium. In the transport protocol, the virtio driver acts as the initiator, while the virtio SCSI host provides one or more targets that receive and process the requests.

5.6.1 Device ID

8

5.6.2 Virtqueues

0

controlq

1

eventq

2…n

request queues

5.6.3 Feature bits

VIRTIO_SCSI_F_INOUT (0)

A single request can include both read-only and write-only data buffers.

VIRTIO_SCSI_F_HOTPLUG (1)

The host should enable hot-plug/hot-unplug of new LUNs and targets on the SCSI bus.

VIRTIO_SCSI_F_CHANGE (2)

The host will report changes to LUN parameters via a VIRTIO_SCSI_T_PARAM_CHANGE event.

5.6.4 Device configuration layout

All fields of this configuration are always available. sense_size and cdb_size are writable by the driver.