Vhost-user Protocol 

Introduction 

This protocol is aiming to complement the ioctl interface used to control the vhost implementation in the Linux kernel. It implements the control plane needed to establish virtqueue sharing with a user space process on the same host. It uses communication over a Unix domain socket to share file descriptors in the ancillary data of the message.

The protocol defines 2 sides of the communication, front-end and back-end. The front-end is the application that shares its virtqueues, in our case QEMU. The back-end is the consumer of the virtqueues.

In the current implementation QEMU is the front-end, and the back-end is the external process consuming the virtio queues, for example a software Ethernet switch running in user space, such as Snabbswitch, or a block device back-end processing read & write to a virtual disk. In order to facilitate interoperability between various back-end implementations, it is recommended to follow the Backend program conventions.

The front-end and back-end can be either a client (i.e. connecting) or server (listening) in the socket communication.

Support for platforms other than Linux 

While vhost-user was initially developed targeting Linux, nowadays it is supported on any platform that provides the following features:

A way for requesting shared memory represented by a file descriptor so it can be passed over a UNIX domain socket and then mapped by the other process.
AF_UNIX sockets with SCM_RIGHTS, so QEMU and the other process can exchange messages through it, including ancillary data when needed.
Either eventfd or pipe/pipe2. On platforms where eventfd is not available, QEMU will automatically fall back to pipe2 or, as a last resort, pipe. Each file descriptor will be used for receiving or sending events by reading or writing (respectively) an 8-byte value to the corresponding it. The 8-value itself has no meaning and should not be interpreted.

Message Specification 

Note

All numbers are in the machine native byte order.

A vhost-user message consists of 3 header fields and a payload.

request

flags

size

payload

Payload 

Depending on the request type, payload can be:

A single 64-bit integer 

u64

u64:: a 64-bit unsigned integer

A vring state description 

index

num

index:: a 32-bit index
num:: a 32-bit number

A vring descriptor index for split virtqueues 

vring index

index in avail ring

vring index:

32-bit index of the respective virtqueue

index in avail ring:

32-bit value, of which currently only the lower 16 bits are used:

Bits 0–15: Index of the next Available Ring descriptor that the back-end will process. This is a free-running index that is not wrapped by the ring size.
Bits 16–31: Reserved (set to zero)

Vring descriptor indices for packed virtqueues 

vring index

descriptor indices

vring index:

32-bit index of the respective virtqueue

descriptor indices:

32-bit value:

Bits 0–14: Index of the next Available Ring descriptor that the back-end will process. This is a free-running index that is not wrapped by the ring size.
Bit 15: Driver (Available) Ring Wrap Counter
Bits 16–30: Index of the entry in the Used Ring where the back-end will place the next descriptor. This is a free-running index that is not wrapped by the ring size.
Bit 31: Device (Used) Ring Wrap Counter

A vring address description 

index

flags

size

descriptor

used

available

log

index:: a 32-bit vring index
flags:: a 32-bit vring flags
descriptor:: a 64-bit ring address of the vring descriptor table
used:: a 64-bit ring address of the vring used ring
available:: a 64-bit ring address of the vring available ring
log:: a 64-bit guest address for logging

Note that a ring address is an IOVA if VIRTIO_F_IOMMU_PLATFORM has been negotiated. Otherwise it is a user address.

Memory region description 

guest address

size

user address

mmap offset

guest address:: a 64-bit guest address of the region
size:: a 64-bit size
user address:: a 64-bit user address
mmap offset:: 64-bit offset where region starts in the mapped memory

When the VHOST_USER_PROTOCOL_F_XEN_MMAP protocol feature has been successfully negotiated, the memory region description contains two extra fields at the end.

guest address

size

user address

mmap offset

xen mmap flags

domid

xen mmap flags:: 32-bit bit field

Bit 0 is set for Xen foreign memory mapping.
Bit 1 is set for Xen grant memory mapping.
Bit 8 is set if the memory region can not be mapped in advance, and memory areas within this region must be mapped / unmapped only when required by the back-end. The back-end shouldn’t try to map the entire region at once, as the front-end may not allow it. The back-end should rather map only the required amount of memory at once and unmap it after it is used.

domid:: a 32-bit Xen hypervisor specific domain id.

Single memory region description 

padding

region

padding:: 64-bit

A region is represented by Memory region description.

Multiple Memory regions description 

num regions

padding

region0

…

region7

num regions:: a 32-bit number of regions
padding:: 32-bit

A region is represented by Memory region description.

Log description 

log size

log offset

log size:: size of area used for logging
log offset:: offset from start of supplied file descriptor where logging starts (i.e. where guest address 0 would be logged)

An IOTLB message 

iova

size

user address

permissions flags

type

iova:: a 64-bit I/O virtual address programmed by the guest
size:: a 64-bit size
user address:: a 64-bit user address
permissions flags:: an 8-bit value: - 0: No access - 1: Read access - 2: Write access - 3: Read/Write access
type:: an 8-bit IOTLB message type: - 1: IOTLB miss - 2: IOTLB update - 3: IOTLB invalidate - 4: IOTLB access fail

Virtio device config space 

offset

size

flags

payload

offset:: a 32-bit offset of virtio device’s configuration space
size:: a 32-bit configuration space access size in bytes
flags:: a 32-bit value: - 0: Vhost front-end messages used for writable fields - 1: Vhost front-end messages used for live migration
payload:: Size bytes array holding the contents of the virtio device’s configuration space

Vring area description 

u64

size

offset

u64:: a 64-bit integer contains vring index and flags
size:: a 64-bit size of this area
offset:: a 64-bit offset of this area from the start of the supplied file descriptor

Inflight description 

mmap size

mmap offset

num queues

queue size

mmap size:: a 64-bit size of area to track inflight I/O
mmap offset:: a 64-bit offset of this area from the start of the supplied file descriptor
num queues:: a 16-bit number of virtqueues
queue size:: a 16-bit size of virtqueues

VhostUserShared 

UUID

UUID:: 16 bytes UUID, whose first three components (a 32-bit value, then two 16-bit values) are stored in big endian.

Device state transfer parameters 

transfer direction

migration phase

transfer direction:

a 32-bit enum, describing the direction in which the state is transferred:

0: Save: Transfer the state from the back-end to the front-end, which happens on the source side of migration
1: Load: Transfer the state from the front-end to the back-end, which happens on the destination side of migration

migration phase:

a 32-bit enum, describing the state in which the VM guest and devices are:

0: Stopped (in the period after the transfer of memory-mapped regions before switch-over to the destination): The VM guest is stopped, and the vhost-user device is suspended (see Suspended device state).

In the future, additional phases might be added e.g. to allow iterative migration while the device is running.

C structure 

In QEMU the vhost-user message is implemented with the following struct:

typedef struct VhostUserMsg {
    VhostUserRequest request;
    uint32_t flags;
    uint32_t size;
    union {
        uint64_t u64;
        struct vhost_vring_state state;
        struct vhost_vring_addr addr;
        VhostUserMemory memory;
        VhostUserLog log;
        struct vhost_iotlb_msg iotlb;
        VhostUserConfig config;
        VhostUserVringArea area;
        VhostUserInflight inflight;
    };
} QEMU_PACKED VhostUserMsg;

Communication 

The protocol for vhost-user is based on the existing implementation of vhost for the Linux Kernel. Most messages that can be sent via the Unix domain socket implementing vhost-user have an equivalent ioctl to the kernel implementation.

The communication consists of the front-end sending message requests and the back-end sending message replies. Most of the requests don’t require replies. Here is a list of the ones that do:

VHOST_USER_GET_FEATURES
VHOST_USER_GET_PROTOCOL_FEATURES
VHOST_USER_GET_VRING_BASE
VHOST_USER_SET_LOG_BASE (if VHOST_USER_PROTOCOL_F_LOG_SHMFD)
VHOST_USER_GET_INFLIGHT_FD (if VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD)

Ring states 

Rings have two independent states: started/stopped, and enabled/disabled.

While a ring is stopped, the back-end must not process the ring at all, regardless of whether it is enabled or disabled. The enabled/disabled state should still be tracked, though, so it can come into effect once the ring is started.
started and disabled: The back-end must process the ring without causing any side effects. For example, for a networking device, in the disabled state the back-end must not supply any new RX packets, but must process and discard any TX packets.
started and enabled: The back-end must process the ring normally, i.e. process all requests and execute them.

Each ring is initialized in a stopped and disabled state. The back-end must start a ring upon receiving a kick (that is, detecting that file descriptor is readable) on the descriptor specified by VHOST_USER_SET_VRING_KICK or receiving the in-band message VHOST_USER_VRING_KICK if negotiated, and stop a ring upon receiving VHOST_USER_GET_VRING_BASE.

Rings can be enabled or disabled by VHOST_USER_SET_VRING_ENABLE.

In addition, upon receiving a VHOST_USER_SET_FEATURES message from the front-end without VHOST_USER_F_PROTOCOL_FEATURES set, the back-end must enable all rings immediately.

While processing the rings (whether they are enabled or not), the back-end must support changing some configuration aspects on the fly.

Suspended device state 

While all vrings are stopped, the device is suspended. In addition to not processing any vring (because they are stopped), the device must:

not write to any guest memory regions,
not send any notifications to the guest,
not send any messages to the front-end,
still process and reply to messages from the front-end.

Multiple queue support 

Many devices have a fixed number of virtqueues. In this case the front-end already knows the number of available virtqueues without communicating with the back-end.

Some devices do not have a fixed number of virtqueues. Instead the maximum number of virtqueues is chosen by the back-end. The number can depend on host resource availability or back-end implementation details. Such devices are called multiple queue devices.

Multiple queue support allows the back-end to advertise the maximum number of queues. This is treated as a protocol extension, hence the back-end has to implement protocol features first. The multiple queues feature is supported only when the protocol feature VHOST_USER_PROTOCOL_F_MQ (bit 0) is set.

The max number of queues the back-end supports can be queried with message VHOST_USER_GET_QUEUE_NUM. Front-end should stop when the number of requested queues is bigger than that.

As all queues share one connection, the front-end uses a unique index for each queue in the sent message to identify a specified queue.

The front-end enables queues by sending message VHOST_USER_SET_VRING_ENABLE. vhost-user-net has historically automatically enabled the first queue pair.

Back-ends should always implement the VHOST_USER_PROTOCOL_F_MQ protocol feature, even for devices with a fixed number of virtqueues, since it is simple to implement and offers a degree of introspection.

Front-ends must not rely on the VHOST_USER_PROTOCOL_F_MQ protocol feature for devices with a fixed number of virtqueues. Only true multiqueue devices require this protocol feature.

Migration 

During live migration, the front-end may need to track the modifications the back-end makes to the memory mapped regions. The front-end should mark the dirty pages in a log. Once it complies to this logging, it may declare the VHOST_F_LOG_ALL vhost feature.

To start/stop logging of data/used ring writes, the front-end may send messages VHOST_USER_SET_FEATURES with VHOST_F_LOG_ALL and VHOST_USER_SET_VRING_ADDR with VHOST_VRING_F_LOG in ring’s flags set to 1/0, respectively.

All the modifications to memory pointed by vring “descriptor” should be marked. Modifications to “used” vring should be marked if VHOST_VRING_F_LOG is part of ring’s flags.

Dirty pages are of size:

#define VHOST_LOG_PAGE 0x1000

The log memory fd is provided in the ancillary data of VHOST_USER_SET_LOG_BASE message when the back-end has VHOST_USER_PROTOCOL_F_LOG_SHMFD protocol feature.

The size of the log is supplied as part of VhostUserMsg which should be large enough to cover all known guest addresses. Log starts at the supplied offset in the supplied file descriptor. The log covers from address 0 to the maximum of guest regions. In pseudo-code, to mark page at addr as dirty:

page = addr / VHOST_LOG_PAGE
log[page / 8] |= 1 << page % 8

Where addr is the guest physical address.

Use atomic operations, as the log may be concurrently manipulated.

Note that when logging modifications to the used ring (when VHOST_VRING_F_LOG is set for this ring), log_guest_addr should be used to calculate the log offset: the write to first byte of the used ring is logged at this offset from log start. Also note that this value might be outside the legal guest physical address range (i.e. does not have to be covered by the VhostUserMemory table), but the bit offset of the last byte of the ring must fall within the size supplied by VhostUserLog.

VHOST_USER_SET_LOG_FD is an optional message with an eventfd in ancillary data, it may be used to inform the front-end that the log has been modified.

Once the source has finished migration, rings will be stopped by the source (Suspended device state). No further update must be done before rings are restarted.

In postcopy migration the back-end is started before all the memory has been received from the source host, and care must be taken to avoid accessing pages that have yet to be received. The back-end opens a ‘userfault’-fd and registers the memory with it; this fd is then passed back over to the front-end. The front-end services requests on the userfaultfd for pages that are accessed and when the page is available it performs WAKE ioctl’s on the userfaultfd to wake the stalled back-end. The front-end indicates support for this via the VHOST_USER_PROTOCOL_F_PAGEFAULT feature.

Migrating back-end state 

Migrating device state involves transferring the state from one back-end, called the source, to another back-end, called the destination. After migration, the destination transparently resumes operation without requiring the driver to re-initialize the device at the VIRTIO level. If the migration fails, then the source can transparently resume operation until another migration attempt is made.

Generally, the front-end is connected to a virtual machine guest (which contains the driver), which has its own state to transfer between source and destination, and therefore will have an implementation-specific mechanism to do so. The VHOST_USER_PROTOCOL_F_DEVICE_STATE feature provides functionality to have the front-end include the back-end’s state in this transfer operation so the back-end does not need to implement its own mechanism, and so the virtual machine may have its complete state, including vhost-user devices’ states, contained within a single stream of data.

To do this, the back-end state is transferred from back-end to front-end on the source side, and vice versa on the destination side. This transfer happens over a channel that is negotiated using the VHOST_USER_SET_DEVICE_STATE_FD message. This message has two parameters:

Direction of transfer: On the source, the data is saved, transferring it from the back-end to the front-end. On the destination, the data is loaded, transferring it from the front-end to the back-end.
Migration phase: Currently, the only supported phase is the period after the transfer of memory-mapped regions before switch-over to the destination, when both the source and destination devices are suspended (Suspended device state). In the future, additional phases might be supported to allow iterative migration while the device is running.

The nature of the channel is implementation-defined, but it must generally behave like a pipe: The writing end will write all the data it has into it, signalling the end of data by closing its end. The reading end must read all of this data (until encountering the end of file) and process it.

When saving, the writing end is the source back-end, and the reading end is the source front-end. After reading the state data from the channel, the source front-end must transfer it to the destination front-end through an implementation-defined mechanism.
When loading, the writing end is the destination front-end, and the reading end is the destination back-end. After reading the state data from the channel, the destination back-end must deserialize its internal state from that data and set itself up to allow the driver to seamlessly resume operation on the VIRTIO level.

Seamlessly resuming operation means that the migration must be transparent to the guest driver, which operates on the VIRTIO level. This driver will not perform any re-initialization steps, but continue to use the device as if no migration had occurred. The vhost-user front-end, however, will re-initialize the vhost state on the destination, following the usual protocol for establishing a connection to a vhost-user back-end: This includes, for example, setting up memory mappings and kick and call FDs as necessary, negotiating protocol features, or setting the initial vring base indices (to the same value as on the source side, so that operation can resume).

Both on the source and on the destination side, after the respective front-end has seen all data transferred (when the transfer FD has been closed), it sends the VHOST_USER_CHECK_DEVICE_STATE message to verify that data transfer was successful in the back-end, too. The back-end responds once it knows whether the transfer and processing was successful or not.

Memory access 

The front-end sends a list of vhost memory regions to the back-end using the VHOST_USER_SET_MEM_TABLE message. Each region has two base addresses: a guest address and a user address.

Messages contain guest addresses and/or user addresses to reference locations within the shared memory. The mapping of these addresses works as follows.

User addresses map to the vhost memory region containing that user address.

When the VIRTIO_F_IOMMU_PLATFORM feature has not been negotiated:

Guest addresses map to the vhost memory region containing that guest address.

When the VIRTIO_F_IOMMU_PLATFORM feature has been negotiated:

Guest addresses are also called I/O virtual addresses (IOVAs). They are translated to user addresses via the IOTLB.
The vhost memory region guest address is not used.

IOMMU support 

When the VIRTIO_F_IOMMU_PLATFORM feature has been negotiated, the front-end sends IOTLB entries update & invalidation by sending VHOST_USER_IOTLB_MSG requests to the back-end with a struct vhost_iotlb_msg as payload. For update events, the iotlb payload has to be filled with the update message type (2), the I/O virtual address, the size, the user virtual address, and the permissions flags. Addresses and size must be within vhost memory regions set via the VHOST_USER_SET_MEM_TABLE request. For invalidation events, the iotlb payload has to be filled with the invalidation message type (3), the I/O virtual address and the size. On success, the back-end is expected to reply with a zero payload, non-zero otherwise.

The back-end relies on the back-end communication channel (see Back-end communication section below) to send IOTLB miss and access failure events, by sending VHOST_USER_BACKEND_IOTLB_MSG requests to the front-end with a struct vhost_iotlb_msg as payload. For miss events, the iotlb payload has to be filled with the miss message type (1), the I/O virtual address and the permissions flags. For access failure event, the iotlb payload has to be filled with the access failure message type (4), the I/O virtual address and the permissions flags. For synchronization purpose, the back-end may rely on the reply-ack feature, so the front-end may send a reply when operation is completed if the reply-ack feature is negotiated and back-ends requests a reply. For miss events, completed operation means either front-end sent an update message containing the IOTLB entry containing requested address and permission, or front-end sent nothing if the IOTLB miss message is invalid (invalid IOVA or permission).

The front-end isn’t expected to take the initiative to send IOTLB update messages, as the back-end sends IOTLB miss messages for the guest virtual memory areas it needs to access.

Back-end communication 

An optional communication channel is provided if the back-end declares VHOST_USER_PROTOCOL_F_BACKEND_REQ protocol feature, to allow the back-end to make requests to the front-end.

The fd is provided via VHOST_USER_SET_BACKEND_REQ_FD ancillary data.

A back-end may then send VHOST_USER_BACKEND_* messages to the front-end using this fd communication channel.

If VHOST_USER_PROTOCOL_F_BACKEND_SEND_FD protocol feature is negotiated, back-end can send file descriptors (at most 8 descriptors in each message) to front-end via ancillary data using this fd communication channel.

Inflight I/O tracking 

To support reconnecting after restart or crash, back-end may need to resubmit inflight I/Os. If virtqueue is processed in order, we can easily achieve that by getting the inflight descriptors from descriptor table (split virtqueue) or descriptor ring (packed virtqueue). However, it can’t work when we process descriptors out-of-order because some entries which store the information of inflight descriptors in available ring (split virtqueue) or descriptor ring (packed virtqueue) might be overridden by new entries. To solve this problem, the back-end need to allocate an extra buffer to store this information of inflight descriptors and share it with front-end for persistent. VHOST_USER_GET_INFLIGHT_FD and VHOST_USER_SET_INFLIGHT_FD are used to transfer this buffer between front-end and back-end. And the format of this buffer is described below:

queue0 region

queue1 region

…

queueN region

N is the number of available virtqueues. The back-end could get it from num queues field of VhostUserInflight.

For split virtqueue, queue region can be implemented as:

typedef struct DescStateSplit {
    /* Indicate whether this descriptor is inflight or not.
     * Only available for head-descriptor. */
    uint8_t inflight;

    /* Padding */
    uint8_t padding[5];

    /* Maintain a list for the last batch of used descriptors.
     * Only available when batching is used for submitting */
    uint16_t next;

    /* Used to preserve the order of fetching available descriptors.
     * Only available for head-descriptor. */
    uint64_t counter;
} DescStateSplit;

typedef struct QueueRegionSplit {
    /* The feature flags of this region. Now it's initialized to 0. */
    uint64_t features;

    /* The version of this region. It's 1 currently.
     * Zero value indicates an uninitialized buffer */
    uint16_t version;

    /* The size of DescStateSplit array. It's equal to the virtqueue size.
     * The back-end could get it from queue size field of VhostUserInflight. */
    uint16_t desc_num;

    /* The head of list that track the last batch of used descriptors. */
    uint16_t last_batch_head;

    /* Store the idx value of used ring */
    uint16_t used_idx;

    /* Used to track the state of each descriptor in descriptor table */
    DescStateSplit desc[];
} QueueRegionSplit;

To track inflight I/O, the queue region should be processed as follows:

When receiving available buffers from the driver:

Get the next available head-descriptor index from available ring, i
Set desc[i].counter to the value of global counter
Increase global counter by 1
Set desc[i].inflight to 1

When supplying used buffers to the driver:

Get corresponding used head-descriptor index, i
Set desc[i].next to last_batch_head
Set last_batch_head to i
Steps 1,2,3 may be performed repeatedly if batching is possible
Increase the idx value of used ring by the size of the batch
Set the inflight field of each DescStateSplit entry in the batch to 0
Set used_idx to the idx value of used ring

When reconnecting:

If the value of used_idx does not match the idx value of used ring (means the inflight field of DescStateSplit entries in last batch may be incorrect),
1. Subtract the value of used_idx from the idx value of used ring to get last batch size of DescStateSplit entries
2. Set the inflight field of each DescStateSplit entry to 0 in last batch list which starts from last_batch_head
3. Set used_idx to the idx value of used ring
Resubmit inflight DescStateSplit entries in order of their counter value

For packed virtqueue, queue region can be implemented as:

typedef struct DescStatePacked {
    /* Indicate whether this descriptor is inflight or not.
     * Only available for head-descriptor. */
    uint8_t inflight;

    /* Padding */
    uint8_t padding;

    /* Link to the next free entry */
    uint16_t next;

    /* Link to the last entry of descriptor list.
     * Only available for head-descriptor. */
    uint16_t last;

    /* The length of descriptor list.
     * Only available for head-descriptor. */
    uint16_t num;

    /* Used to preserve the order of fetching available descriptors.
     * Only available for head-descriptor. */
    uint64_t counter;

    /* The buffer id */
    uint16_t id;

    /* The descriptor flags */
    uint16_t flags;

    /* The buffer length */
    uint32_t len;

    /* The buffer address */
    uint64_t addr;
} DescStatePacked;

typedef struct QueueRegionPacked {
    /* The feature flags of this region. Now it's initialized to 0. */
    uint64_t features;

    /* The version of this region. It's 1 currently.
     * Zero value indicates an uninitialized buffer */
    uint16_t version;

    /* The size of DescStatePacked array. It's equal to the virtqueue size.
     * The back-end could get it from queue size field of VhostUserInflight. */
    uint16_t desc_num;

    /* The head of free DescStatePacked entry list */
    uint16_t free_head;

    /* The old head of free DescStatePacked entry list */
    uint16_t old_free_head;

    /* The used index of descriptor ring */
    uint16_t used_idx;

    /* The old used index of descriptor ring */
    uint16_t old_used_idx;

    /* Device ring wrap counter */
    uint8_t used_wrap_counter;

    /* The old device ring wrap counter */
    uint8_t old_used_wrap_counter;

    /* Padding */
    uint8_t padding[7];

    /* Used to track the state of each descriptor fetched from descriptor ring */
    DescStatePacked desc[];
} QueueRegionPacked;

To track inflight I/O, the queue region should be processed as follows:

When receiving available buffers from the driver:

Get the next available descriptor entry from descriptor ring, d
If d is head descriptor,
1. Set desc[old_free_head].num to 0
2. Set desc[old_free_head].counter to the value of global counter
3. Increase global counter by 1
4. Set desc[old_free_head].inflight to 1
If d is last descriptor, set desc[old_free_head].last to free_head
Increase desc[old_free_head].num by 1
Set desc[free_head].addr, desc[free_head].len, desc[free_head].flags, desc[free_head].id to d.addr, d.len, d.flags, d.id
Set free_head to desc[free_head].next
If d is last descriptor, set old_free_head to free_head

When supplying used buffers to the driver:

Get corresponding used head-descriptor entry from descriptor ring, d
Get corresponding DescStatePacked entry, e
Set desc[e.last].next to free_head
Set free_head to the index of e
Steps 1,2,3,4 may be performed repeatedly if batching is possible
Increase used_idx by the size of the batch and update used_wrap_counter if needed
Update d.flags
Set the inflight field of each head DescStatePacked entry in the batch to 0
Set old_free_head, old_used_idx, old_used_wrap_counter to free_head, used_idx, used_wrap_counter

When reconnecting:

If used_idx does not match old_used_idx (means the inflight field of DescStatePacked entries in last batch may be incorrect),
1. Get the next descriptor ring entry through old_used_idx, d
2. Use old_used_wrap_counter to calculate the available flags
3. If d.flags is not equal to the calculated flags value (means back-end has submitted the buffer to guest driver before crash, so it has to commit the in-progres update), set old_free_head, old_used_idx, old_used_wrap_counter to free_head, used_idx, used_wrap_counter
Set free_head, used_idx, used_wrap_counter to old_free_head, old_used_idx, old_used_wrap_counter (roll back any in-progress update)
Set the inflight field of each DescStatePacked entry in free list to 0
Resubmit inflight DescStatePacked entries in order of their counter value

In-band notifications 

In some limited situations (e.g. for simulation) it is desirable to have the kick, call and error (if used) signals done via in-band messages instead of asynchronous eventfd notifications. This can be done by negotiating the VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS protocol feature.

Note that due to the fact that too many messages on the sockets can cause the sending application(s) to block, it is not advised to use this feature unless absolutely necessary. It is also considered an error to negotiate this feature without also negotiating VHOST_USER_PROTOCOL_F_BACKEND_REQ and VHOST_USER_PROTOCOL_F_REPLY_ACK, the former is necessary for getting a message channel from the back-end to the front-end, while the latter needs to be used with the in-band notification messages to block until they are processed, both to avoid blocking later and for proper processing (at least in the simulation use case.) As it has no other way of signalling this error, the back-end should close the connection as a response to a VHOST_USER_SET_PROTOCOL_FEATURES message that sets the in-band notifications feature flag without the other two.

Protocol features 

#define VHOST_USER_PROTOCOL_F_MQ                    0
#define VHOST_USER_PROTOCOL_F_LOG_SHMFD             1
#define VHOST_USER_PROTOCOL_F_RARP                  2
#define VHOST_USER_PROTOCOL_F_REPLY_ACK             3
#define VHOST_USER_PROTOCOL_F_MTU                   4
#define VHOST_USER_PROTOCOL_F_BACKEND_REQ           5
#define VHOST_USER_PROTOCOL_F_CROSS_ENDIAN          6
#define VHOST_USER_PROTOCOL_F_CRYPTO_SESSION        7
#define VHOST_USER_PROTOCOL_F_PAGEFAULT             8
#define VHOST_USER_PROTOCOL_F_CONFIG                9
#define VHOST_USER_PROTOCOL_F_BACKEND_SEND_FD      10
#define VHOST_USER_PROTOCOL_F_HOST_NOTIFIER        11
#define VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD       12
#define VHOST_USER_PROTOCOL_F_RESET_DEVICE         13
#define VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS 14
#define VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS  15
#define VHOST_USER_PROTOCOL_F_STATUS               16
#define VHOST_USER_PROTOCOL_F_XEN_MMAP             17
#define VHOST_USER_PROTOCOL_F_SHARED_OBJECT        18
#define VHOST_USER_PROTOCOL_F_DEVICE_STATE         19

Front-end message types 

VHOST_USER_GET_FEATURES

id:: 1
equivalent ioctl:: VHOST_GET_FEATURES
request payload:: N/A
reply payload:: u64

Get from the underlying vhost implementation the features bitmask. Feature bit VHOST_USER_F_PROTOCOL_FEATURES signals back-end support for VHOST_USER_GET_PROTOCOL_FEATURES and VHOST_USER_SET_PROTOCOL_FEATURES.

VHOST_USER_SET_FEATURES

id:: 2
equivalent ioctl:: VHOST_SET_FEATURES
request payload:: u64
reply payload:: N/A

Enable features in the underlying vhost implementation using a bitmask. Feature bit VHOST_USER_F_PROTOCOL_FEATURES signals back-end support for VHOST_USER_GET_PROTOCOL_FEATURES and VHOST_USER_SET_PROTOCOL_FEATURES.

VHOST_USER_GET_PROTOCOL_FEATURES

id:: 15
equivalent ioctl:: VHOST_GET_FEATURES
request payload:: N/A
reply payload:: u64

Get the protocol feature bitmask from the underlying vhost implementation. Only legal if feature bit VHOST_USER_F_PROTOCOL_FEATURES is present in VHOST_USER_GET_FEATURES. It does not need to be acknowledged by VHOST_USER_SET_FEATURES.

Note

Back-ends that report VHOST_USER_F_PROTOCOL_FEATURES must support this message even before VHOST_USER_SET_FEATURES was called.

VHOST_USER_SET_PROTOCOL_FEATURES

id:: 16
equivalent ioctl:: VHOST_SET_FEATURES
request payload:: u64
reply payload:: N/A

Enable protocol features in the underlying vhost implementation.

Only legal if feature bit VHOST_USER_F_PROTOCOL_FEATURES is present in VHOST_USER_GET_FEATURES. It does not need to be acknowledged by VHOST_USER_SET_FEATURES.

Note

Back-ends that report VHOST_USER_F_PROTOCOL_FEATURES must support this message even before VHOST_USER_SET_FEATURES was called.

VHOST_USER_SET_OWNER

id:: 3
equivalent ioctl:: VHOST_SET_OWNER
request payload:: N/A
reply payload:: N/A

Issued when a new connection is established. It marks the sender as the front-end that owns of the session. This can be used on the back-end as a “session start” flag.

VHOST_USER_RESET_OWNER

id:: 4
request payload:: N/A
reply payload:: N/A

Deprecated

This is no longer used. Used to be sent to request disabling all rings, but some back-ends interpreted it to also discard connection state (this interpretation would lead to bugs). It is recommended that back-ends either ignore this message, or use it to disable all rings.

VHOST_USER_SET_MEM_TABLE

id:: 5
equivalent ioctl:: VHOST_SET_MEM_TABLE
request payload:: multiple memory regions description
reply payload:: (postcopy only) multiple memory regions description

Sets the memory map regions on the back-end so it can translate the vring addresses. In the ancillary data there is an array of file descriptors for each memory mapped region. The size and ordering of the fds matches the number and ordering of memory regions.

When VHOST_USER_POSTCOPY_LISTEN has been received, SET_MEM_TABLE replies with the bases of the memory mapped regions to the front-end. The back-end must have mmap’d the regions but not yet accessed them and should not yet generate a userfault event.

Note

NEED_REPLY_MASK is not set in this case. QEMU will then reply back to the list of mappings with an empty VHOST_USER_SET_MEM_TABLE as an acknowledgement; only upon reception of this message may the guest start accessing the memory and generating faults.

VHOST_USER_SET_LOG_BASE

id:: 6
equivalent ioctl:: VHOST_SET_LOG_BASE
request payload:: u64
reply payload:: N/A

Sets logging shared memory space.

When the back-end has VHOST_USER_PROTOCOL_F_LOG_SHMFD protocol feature, the log memory fd is provided in the ancillary data of VHOST_USER_SET_LOG_BASE message, the size and offset of shared memory area provided in the message.

VHOST_USER_SET_LOG_FD

id:: 7
equivalent ioctl:: VHOST_SET_LOG_FD
request payload:: N/A
reply payload:: N/A

Sets the logging file descriptor, which is passed as ancillary data.

VHOST_USER_SET_VRING_NUM

id:: 8
equivalent ioctl:: VHOST_SET_VRING_NUM
request payload:: vring state description
reply payload:: N/A

Set the size of the queue.

VHOST_USER_SET_VRING_ADDR

id:: 9
equivalent ioctl:: VHOST_SET_VRING_ADDR
request payload:: vring address description
reply payload:: N/A

Sets the addresses of the different aspects of the vring.

VHOST_USER_SET_VRING_BASE

id:: 10
equivalent ioctl:: VHOST_SET_VRING_BASE
request payload:: vring descriptor index/indices
reply payload:: N/A

Sets the next index to use for descriptors in this vring:

For a split virtqueue, sets only the next descriptor index to process in the Available Ring. The device is supposed to read the next index in the Used Ring from the respective vring structure in guest memory.
For a packed virtqueue, both indices are supplied, as they are not explicitly available in memory.

Consequently, the payload type is specific to the type of virt queue (a vring descriptor index for split virtqueues vs. vring descriptor indices for packed virtqueues).

VHOST_USER_GET_VRING_BASE

id:: 11
equivalent ioctl:: VHOST_USER_GET_VRING_BASE
request payload:: vring state description
reply payload:: vring descriptor index/indices

Stops the vring and returns the current descriptor index or indices:

For a split virtqueue, returns only the 16-bit next descriptor index to process in the Available Ring. Note that this may differ from the available ring index in the vring structure in memory, which points to where the driver will put new available descriptors. For the Used Ring, the device only needs the next descriptor index at which to put new descriptors, which is the value in the vring structure in memory, so this value is not covered by this message.

For a packed virtqueue, neither index is explicitly available to read from memory, so both indices (as maintained by the device) are returned.

Consequently, the payload type is specific to the type of virt queue (a vring descriptor index for split virtqueues vs. vring descriptor indices for packed virtqueues).

When and as long as all of a device’s vrings are stopped, it is suspended, see Suspended device state.

The request payload’s num field is currently reserved and must be set to 0.

VHOST_USER_SET_VRING_KICK

id:: 12
equivalent ioctl:: VHOST_SET_VRING_KICK
request payload:: u64
reply payload:: N/A

Set the event file descriptor for adding buffers to the vring. It is passed in the ancillary data.

Bits (0-7) of the payload contain the vring index. Bit 8 is the invalid FD flag. This flag is set when there is no file descriptor in the ancillary data. This signals that polling should be used instead of waiting for the kick. Note that if the protocol feature VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS has been negotiated this message isn’t necessary as the ring is also started on the VHOST_USER_VRING_KICK message, it may however still be used to set an event file descriptor (which will be preferred over the message) or to enable polling.

VHOST_USER_SET_VRING_CALL

id:: 13
equivalent ioctl:: VHOST_SET_VRING_CALL
request payload:: u64
reply payload:: N/A

Set the event file descriptor to signal when buffers are used. It is passed in the ancillary data.

Bits (0-7) of the payload contain the vring index. Bit 8 is the invalid FD flag. This flag is set when there is no file descriptor in the ancillary data. This signals that polling will be used instead of waiting for the call. Note that if the protocol features VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS and VHOST_USER_PROTOCOL_F_BACKEND_REQ have been negotiated this message isn’t necessary as the VHOST_USER_BACKEND_VRING_CALL message can be used, it may however still be used to set an event file descriptor or to enable polling.

VHOST_USER_SET_VRING_ERR

id:: 14
equivalent ioctl:: VHOST_SET_VRING_ERR
request payload:: u64
reply payload:: N/A

Set the event file descriptor to signal when error occurs. It is passed in the ancillary data.

Bits (0-7) of the payload contain the vring index. Bit 8 is the invalid FD flag. This flag is set when there is no file descriptor in the ancillary data. Note that if the protocol features VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS and VHOST_USER_PROTOCOL_F_BACKEND_REQ have been negotiated this message isn’t necessary as the VHOST_USER_BACKEND_VRING_ERR message can be used, it may however still be used to set an event file descriptor (which will be preferred over the message).

VHOST_USER_GET_QUEUE_NUM

id:: 17
equivalent ioctl:: N/A
request payload:: N/A
reply payload:: u64

Query how many queues the back-end supports.

This request should be sent only when VHOST_USER_PROTOCOL_F_MQ is set in queried protocol features by VHOST_USER_GET_PROTOCOL_FEATURES.

VHOST_USER_SET_VRING_ENABLE

id:: 18
equivalent ioctl:: N/A
request payload:: vring state description
reply payload:: N/A

Signal the back-end to enable or disable corresponding vring.

This request should be sent only when VHOST_USER_F_PROTOCOL_FEATURES has been negotiated.

VHOST_USER_SEND_RARP

id:: 19
equivalent ioctl:: N/A
request payload:: u64
reply payload:: N/A

Ask vhost user back-end to broadcast a fake RARP to notify the migration is terminated for guest that does not support GUEST_ANNOUNCE.

Only legal if feature bit VHOST_USER_F_PROTOCOL_FEATURES is present in VHOST_USER_GET_FEATURES and protocol feature bit VHOST_USER_PROTOCOL_F_RARP is present in VHOST_USER_GET_PROTOCOL_FEATURES. The first 6 bytes of the payload contain the mac address of the guest to allow the vhost user back-end to construct and broadcast the fake RARP.

VHOST_USER_NET_SET_MTU

id:: 20
equivalent ioctl:: N/A
request payload:: u64
reply payload:: N/A

Set host MTU value exposed to the guest.

This request should be sent only when VIRTIO_NET_F_MTU feature has been successfully negotiated, VHOST_USER_F_PROTOCOL_FEATURES is present in VHOST_USER_GET_FEATURES and protocol feature bit VHOST_USER_PROTOCOL_F_NET_MTU is present in VHOST_USER_GET_PROTOCOL_FEATURES.

If VHOST_USER_PROTOCOL_F_REPLY_ACK is negotiated, the back-end must respond with zero in case the specified MTU is valid, or non-zero otherwise.

VHOST_USER_SET_BACKEND_REQ_FD (previous name VHOST_USER_SET_SLAVE_REQ_FD)

id:: 21
equivalent ioctl:: N/A
request payload:: N/A
reply payload:: N/A

Set the socket file descriptor for back-end initiated requests. It is passed in the ancillary data.

This request should be sent only when VHOST_USER_F_PROTOCOL_FEATURES has been negotiated, and protocol feature bit VHOST_USER_PROTOCOL_F_BACKEND_REQ bit is present in VHOST_USER_GET_PROTOCOL_FEATURES. If VHOST_USER_PROTOCOL_F_REPLY_ACK is negotiated, the back-end must respond with zero for success, non-zero otherwise.

VHOST_USER_IOTLB_MSG

id:: 22
equivalent ioctl:: N/A (equivalent to VHOST_IOTLB_MSG message type)
request payload:: struct vhost_iotlb_msg
reply payload:: u64

Send IOTLB messages with struct vhost_iotlb_msg as payload.

The front-end sends such requests to update and invalidate entries in the device IOTLB. The back-end has to acknowledge the request with sending zero as u64 payload for success, non-zero otherwise.

This request should be send only when VIRTIO_F_IOMMU_PLATFORM feature has been successfully negotiated.

VHOST_USER_SET_VRING_ENDIAN

id:: 23
equivalent ioctl:: VHOST_SET_VRING_ENDIAN
request payload:: vring state description
reply payload:: N/A

Set the endianness of a VQ for legacy devices. Little-endian is indicated with state.num set to 0 and big-endian is indicated with state.num set to 1. Other values are invalid.

This request should be sent only when VHOST_USER_PROTOCOL_F_CROSS_ENDIAN has been negotiated. Backends that negotiated this feature should handle both endiannesses and expect this message once (per VQ) during device configuration (ie. before the front-end starts the VQ).

VHOST_USER_GET_CONFIG

id:: 24
equivalent ioctl:: N/A
request payload:: virtio device config space
reply payload:: virtio device config space

When VHOST_USER_PROTOCOL_F_CONFIG is negotiated, this message is submitted by the vhost-user front-end to fetch the contents of the virtio device configuration space, vhost-user back-end’s payload size MUST match the front-end’s request, vhost-user back-end uses zero length of payload to indicate an error to the vhost-user front-end. The vhost-user front-end may cache the contents to avoid repeated VHOST_USER_GET_CONFIG calls.

VHOST_USER_SET_CONFIG

id:: 25
equivalent ioctl:: N/A
request payload:: virtio device config space
reply payload:: N/A

When VHOST_USER_PROTOCOL_F_CONFIG is negotiated, this message is submitted by the vhost-user front-end when the Guest changes the virtio device configuration space and also can be used for live migration on the destination host. The vhost-user back-end must check the flags field, and back-ends MUST NOT accept SET_CONFIG for read-only configuration space fields unless the live migration bit is set.

VHOST_USER_CREATE_CRYPTO_SESSION

id:: 26
equivalent ioctl:: N/A
request payload:: crypto session description
reply payload:: crypto session description

Create a session for crypto operation. The back-end must return the session id, 0 or positive for success, negative for failure. This request should be sent only when VHOST_USER_PROTOCOL_F_CRYPTO_SESSION feature has been successfully negotiated. It’s a required feature for crypto devices.

VHOST_USER_CLOSE_CRYPTO_SESSION

id:: 27
equivalent ioctl:: N/A
request payload:: u64
reply payload:: N/A

Close a session for crypto operation which was previously created by VHOST_USER_CREATE_CRYPTO_SESSION.

This request should be sent only when VHOST_USER_PROTOCOL_F_CRYPTO_SESSION feature has been successfully negotiated. It’s a required feature for crypto devices.

VHOST_USER_POSTCOPY_ADVISE

id:: 28
request payload:: N/A
reply payload:: userfault fd

When VHOST_USER_PROTOCOL_F_PAGEFAULT is supported, the front-end advises back-end that a migration with postcopy enabled is underway, the back-end must open a userfaultfd for later use. Note that at this stage the migration is still in precopy mode.

VHOST_USER_POSTCOPY_LISTEN

id:: 29
request payload:: N/A
reply payload:: N/A

The front-end advises back-end that a transition to postcopy mode has happened. The back-end must ensure that shared memory is registered with userfaultfd to cause faulting of non-present pages.

This is always sent sometime after a VHOST_USER_POSTCOPY_ADVISE, and thus only when VHOST_USER_PROTOCOL_F_PAGEFAULT is supported.

VHOST_USER_POSTCOPY_END

id:: 30
request payload:: N/A
reply payload:: u64

The front-end advises that postcopy migration has now completed. The back-end must disable the userfaultfd. The reply is an acknowledgement only.

When VHOST_USER_PROTOCOL_F_PAGEFAULT is supported, this message is sent at the end of the migration, after VHOST_USER_POSTCOPY_LISTEN was previously sent.

The value returned is an error indication; 0 is success.

VHOST_USER_GET_INFLIGHT_FD

id:: 31
equivalent ioctl:: N/A
request payload:: inflight description
reply payload:: N/A

When VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD protocol feature has been successfully negotiated, this message is submitted by the front-end to get a shared buffer from back-end. The shared buffer will be used to track inflight I/O by back-end. QEMU should retrieve a new one when vm reset.

VHOST_USER_SET_INFLIGHT_FD

id:: 32
equivalent ioctl:: N/A
request payload:: inflight description
reply payload:: N/A

When VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD protocol feature has been successfully negotiated, this message is submitted by the front-end to send the shared inflight buffer back to the back-end so that the back-end could get inflight I/O after a crash or restart.

VHOST_USER_GPU_SET_SOCKET

id:: 33
equivalent ioctl:: N/A
request payload:: N/A
reply payload:: N/A

Sets the GPU protocol socket file descriptor, which is passed as ancillary data. The GPU protocol is used to inform the front-end of rendering state and updates. See vhost-user-gpu.rst for details.

VHOST_USER_RESET_DEVICE

id:: 34
equivalent ioctl:: N/A
request payload:: N/A
reply payload:: N/A

Ask the vhost user back-end to disable all rings and reset all internal device state to the initial state, ready to be reinitialized. The back-end retains ownership of the device throughout the reset operation.

Only valid if the VHOST_USER_PROTOCOL_F_RESET_DEVICE protocol feature is set by the back-end.

VHOST_USER_VRING_KICK

id:: 35
equivalent ioctl:: N/A
request payload:: vring state description
reply payload:: N/A

When the VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS protocol feature has been successfully negotiated, this message may be submitted by the front-end to indicate that a buffer was added to the vring instead of signalling it using the vring’s kick file descriptor or having the back-end rely on polling.

The state.num field is currently reserved and must be set to 0.

VHOST_USER_GET_MAX_MEM_SLOTS

id:: 36
equivalent ioctl:: N/A
request payload:: N/A
reply payload:: u64

When the VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS protocol feature has been successfully negotiated, this message is submitted by the front-end to the back-end. The back-end should return the message with a u64 payload containing the maximum number of memory slots for QEMU to expose to the guest. The value returned by the back-end will be capped at the maximum number of ram slots which can be supported by the target platform.

VHOST_USER_ADD_MEM_REG

id:: 37
equivalent ioctl:: N/A
request payload:: N/A
reply payload:: single memory region description

When the VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS protocol feature has been successfully negotiated, this message is submitted by the front-end to the back-end. The message payload contains a memory region descriptor struct, describing a region of guest memory which the back-end device must map in. When the VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS protocol feature has been successfully negotiated, along with the VHOST_USER_REM_MEM_REG message, this message is used to set and update the memory tables of the back-end device.

Exactly one file descriptor from which the memory is mapped is passed in the ancillary data.

In postcopy mode (see VHOST_USER_POSTCOPY_LISTEN), the back-end replies with the bases of the memory mapped region to the front-end. For further details on postcopy, see VHOST_USER_SET_MEM_TABLE. They apply to VHOST_USER_ADD_MEM_REG accordingly.

VHOST_USER_REM_MEM_REG

id:: 38
equivalent ioctl:: N/A
request payload:: N/A
reply payload:: single memory region description

When the VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS protocol feature has been successfully negotiated, this message is submitted by the front-end to the back-end. The message payload contains a memory region descriptor struct, describing a region of guest memory which the back-end device must unmap. When the VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS protocol feature has been successfully negotiated, along with the VHOST_USER_ADD_MEM_REG message, this message is used to set and update the memory tables of the back-end device.

The memory region to be removed is identified by its guest address, user address and size. The mmap offset is ignored.

No file descriptors SHOULD be passed in the ancillary data. For compatibility with existing incorrect implementations, the back-end MAY accept messages with one file descriptor. If a file descriptor is passed, the back-end MUST close it without using it otherwise.

VHOST_USER_SET_STATUS

id:: 39
equivalent ioctl:: VHOST_VDPA_SET_STATUS
request payload:: u64
reply payload:: N/A

When the VHOST_USER_PROTOCOL_F_STATUS protocol feature has been successfully negotiated, this message is submitted by the front-end to notify the back-end with updated device status as defined in the Virtio specification.

VHOST_USER_GET_STATUS

id:: 40
equivalent ioctl:: VHOST_VDPA_GET_STATUS
request payload:: N/A
reply payload:: u64

When the VHOST_USER_PROTOCOL_F_STATUS protocol feature has been successfully negotiated, this message is submitted by the front-end to query the back-end for its device status as defined in the Virtio specification.

VHOST_USER_GET_SHARED_OBJECT

id:: 41
equivalent ioctl:: N/A
request payload:: struct VhostUserShared
reply payload:: dmabuf fd

When the VHOST_USER_PROTOCOL_F_SHARED_OBJECT protocol feature has been successfully negotiated, and the UUID is found in the exporters cache, this message is submitted by the front-end to retrieve a given dma-buf fd from a given back-end, determined by the requested UUID. Back-end will reply passing the fd when the operation is successful, or no fd otherwise.

VHOST_USER_SET_DEVICE_STATE_FD

id:: 42
equivalent ioctl:: N/A
request payload:: device state transfer parameters
reply payload:: u64

Front-end and back-end negotiate a channel over which to transfer the back-end’s internal state during migration. Either side (front-end or back-end) may create the channel. The nature of this channel is not restricted or defined in this document, but whichever side creates it must create a file descriptor that is provided to the respectively other side, allowing access to the channel. This FD must behave as follows:

For the writing end, it must allow writing the whole back-end state sequentially. Closing the file descriptor signals the end of transfer.
For the reading end, it must allow reading the whole back-end state sequentially. The end of file signals the end of the transfer.

For example, the channel may be a pipe, in which case the two ends of the pipe fulfill these requirements respectively.

Initially, the front-end creates a channel along with such an FD. It passes the FD to the back-end as ancillary data of a VHOST_USER_SET_DEVICE_STATE_FD message. The back-end may create a different transfer channel, passing the respective FD back to the front-end as ancillary data of the reply. If so, the front-end must then discard its channel and use the one provided by the back-end.

Whether the back-end should decide to use its own channel is decided based on efficiency: If the channel is a pipe, both ends will most likely need to copy data into and out of it. Any channel that allows for more efficient processing on at least one end, e.g. through zero-copy, is considered more efficient and thus preferred. If the back-end can provide such a channel, it should decide to use it.

The request payload contains parameters for the subsequent data transfer, as described in the Migrating back-end state section.

The value returned is both an indication for success, and whether a file descriptor for a back-end-provided channel is returned: Bits 0–7 are 0 on success, and non-zero on error. Bit 8 is the invalid FD flag; this flag is set when there is no file descriptor returned. When this flag is not set, the front-end must use the returned file descriptor as its end of the transfer channel. The back-end must not both indicate an error and return a file descriptor.

Using this function requires prior negotiation of the VHOST_USER_PROTOCOL_F_DEVICE_STATE feature.

VHOST_USER_CHECK_DEVICE_STATE

id:: 43
equivalent ioctl:: N/A
request payload:: N/A
reply payload:: u64

After transferring the back-end’s internal state during migration (see the Migrating back-end state section), check whether the back-end was able to successfully fully process the state.

The value returned indicates success or error; 0 is success, any non-zero value is an error.

Using this function requires prior negotiation of the VHOST_USER_PROTOCOL_F_DEVICE_STATE feature.

Back-end message types 

For this type of message, the request is sent by the back-end and the reply is sent by the front-end.

VHOST_USER_BACKEND_IOTLB_MSG (previous name VHOST_USER_SLAVE_IOTLB_MSG)

id:: 1
equivalent ioctl:: N/A (equivalent to VHOST_IOTLB_MSG message type)
request payload:: struct vhost_iotlb_msg
reply payload:: N/A

Send IOTLB messages with struct vhost_iotlb_msg as payload. The back-end sends such requests to notify of an IOTLB miss, or an IOTLB access failure. If VHOST_USER_PROTOCOL_F_REPLY_ACK is negotiated, and back-end set the VHOST_USER_NEED_REPLY flag, the front-end must respond with zero when operation is successfully completed, or non-zero otherwise. This request should be send only when VIRTIO_F_IOMMU_PLATFORM feature has been successfully negotiated.

VHOST_USER_BACKEND_CONFIG_CHANGE_MSG (previous name VHOST_USER_SLAVE_CONFIG_CHANGE_MSG)

id:: 2
equivalent ioctl:: N/A
request payload:: N/A
reply payload:: N/A

When VHOST_USER_PROTOCOL_F_CONFIG is negotiated, vhost-user back-end sends such messages to notify that the virtio device’s configuration space has changed, for those host devices which can support such feature, host driver can send VHOST_USER_GET_CONFIG message to the back-end to get the latest content. If VHOST_USER_PROTOCOL_F_REPLY_ACK is negotiated, and the back-end sets the VHOST_USER_NEED_REPLY flag, the front-end must respond with zero when operation is successfully completed, or non-zero otherwise.

VHOST_USER_BACKEND_VRING_HOST_NOTIFIER_MSG (previous name VHOST_USER_SLAVE_VRING_HOST_NOTIFIER_MSG)

id:: 3
equivalent ioctl:: N/A
request payload:: vring area description
reply payload:: N/A

Sets host notifier for a specified queue. The queue index is contained in the u64 field of the vring area description. The host notifier is described by the file descriptor (typically it’s a VFIO device fd) which is passed as ancillary data and the size (which is mmap size and should be the same as host page size) and offset (which is mmap offset) carried in the vring area description. QEMU can mmap the file descriptor based on the size and offset to get a memory range. Registering a host notifier means mapping this memory range to the VM as the specified queue’s notify MMIO region. The back-end sends this request to tell QEMU to de-register the existing notifier if any and register the new notifier if the request is sent with a file descriptor.

This request should be sent only when VHOST_USER_PROTOCOL_F_HOST_NOTIFIER protocol feature has been successfully negotiated.

VHOST_USER_BACKEND_VRING_CALL (previous name VHOST_USER_SLAVE_VRING_CALL)

id:: 4
equivalent ioctl:: N/A
request payload:: vring state description
reply payload:: N/A

When the VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS protocol feature has been successfully negotiated, this message may be submitted by the back-end to indicate that a buffer was used from the vring instead of signalling this using the vring’s call file descriptor or having the front-end relying on polling.

The state.num field is currently reserved and must be set to 0.

VHOST_USER_BACKEND_VRING_ERR (previous name VHOST_USER_SLAVE_VRING_ERR)

id:: 5
equivalent ioctl:: N/A
request payload:: vring state description
reply payload:: N/A

When the VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS protocol feature has been successfully negotiated, this message may be submitted by the back-end to indicate that an error occurred on the specific vring, instead of signalling the error file descriptor set by the front-end via VHOST_USER_SET_VRING_ERR.

The state.num field is currently reserved and must be set to 0.

VHOST_USER_BACKEND_SHARED_OBJECT_ADD

id:: 6
equivalent ioctl:: N/A
request payload:: struct VhostUserShared
reply payload:: N/A

When the VHOST_USER_PROTOCOL_F_SHARED_OBJECT protocol feature has been successfully negotiated, this message can be submitted by the backends to add themselves as exporters to the virtio shared lookup table. The back-end device gets associated with a UUID in the shared table. The back-end is responsible of keeping its own table with exported dma-buf fds. When another back-end tries to import the resource associated with the UUID, it will send a message to the front-end, which will act as a proxy to the exporter back-end. If VHOST_USER_PROTOCOL_F_REPLY_ACK is negotiated, and the back-end sets the VHOST_USER_NEED_REPLY flag, the front-end must respond with zero when operation is successfully completed, or non-zero otherwise.

VHOST_USER_BACKEND_SHARED_OBJECT_REMOVE

id:: 7
equivalent ioctl:: N/A
request payload:: struct VhostUserShared
reply payload:: N/A

When the VHOST_USER_PROTOCOL_F_SHARED_OBJECT protocol feature has been successfully negotiated, this message can be submitted by the backend to remove themselves from to the virtio-dmabuf shared table API. The shared table will remove the back-end device associated with the UUID. If VHOST_USER_PROTOCOL_F_REPLY_ACK is negotiated, and the back-end sets the VHOST_USER_NEED_REPLY flag, the front-end must respond with zero when operation is successfully completed, or non-zero otherwise.

VHOST_USER_BACKEND_SHARED_OBJECT_LOOKUP

id:: 8
equivalent ioctl:: N/A
request payload:: struct VhostUserShared
reply payload:: dmabuf fd and u64

When the VHOST_USER_PROTOCOL_F_SHARED_OBJECT protocol feature has been successfully negotiated, this message can be submitted by the backends to retrieve a given dma-buf fd from the virtio-dmabuf shared table given a UUID. Frontend will reply passing the fd and a zero when the operation is successful, or non-zero otherwise. Note that if the operation fails, no fd is sent to the backend.

VHOST_USER_PROTOCOL_F_REPLY_ACK 

The original vhost-user specification only demands replies for certain commands. This differs from the vhost protocol implementation where commands are sent over an ioctl() call and block until the back-end has completed.

With this protocol extension negotiated, the sender (QEMU) can set the need_reply [Bit 3] flag to any command. This indicates that the back-end MUST respond with a Payload VhostUserMsg indicating success or failure. The payload should be set to zero on success or non-zero on failure, unless the message already has an explicit reply body.

The reply payload gives QEMU a deterministic indication of the result of the command. Today, QEMU is expected to terminate the main vhost-user loop upon receiving such errors. In future, qemu could be taught to be more resilient for selective requests.

For the message types that already solicit a reply from the back-end, the presence of VHOST_USER_PROTOCOL_F_REPLY_ACK or need_reply bit being set brings no behavioural change. (See the Communication section for details.)

Backend program conventions 

vhost-user back-ends can provide various devices & services and may need to be configured manually depending on the use case. However, it is a good idea to follow the conventions listed here when possible. Users, QEMU or libvirt, can then rely on some common behaviour to avoid heterogeneous configuration and management of the back-end programs and facilitate interoperability.

Each back-end installed on a host system should come with at least one JSON file that conforms to the vhost-user.json schema. Each file informs the management applications about the back-end type, and binary location. In addition, it defines rules for management apps for picking the highest priority back-end when multiple match the search criteria (see @VhostUserBackend documentation in the schema file).

If the back-end is not capable of enabling a requested feature on the host (such as 3D acceleration with virgl), or the initialization failed, the back-end should fail to start early and exit with a status != 0. It may also print a message to stderr for further details.

The back-end program must not daemonize itself, but it may be daemonized by the management layer. It may also have a restricted access to the system.

File descriptors 0, 1 and 2 will exist, and have regular stdin/stdout/stderr usage (they may have been redirected to /dev/null by the management layer, or to a log handler).

The back-end program must end (as quickly and cleanly as possible) when the SIGTERM signal is received. Eventually, it may receive SIGKILL by the management layer after a few seconds.

The following command line options have an expected behaviour. They are mandatory, unless explicitly said differently:

--socket-path=PATH: This option specify the location of the vhost-user Unix domain socket. It is incompatible with –fd.
--fd=FDNUM: When this argument is given, the back-end program is started with the vhost-user socket as file descriptor FDNUM. It is incompatible with –socket-path.
--print-capabilities: Output to stdout the back-end capabilities in JSON format, and then exit successfully. Other options and arguments should be ignored, and the back-end program should not perform its normal function. The capabilities can be reported dynamically depending on the host capabilities.

The JSON output is described in the vhost-user.json schema, by `@VHostUserBackendCapabilities. Example:

{
  "type": "foo",
  "features": [
    "feature-a",
    "feature-b"
  ]
}

vhost-user-input 

Command line options:

--evdev-path=PATH

Specify the linux input device.

(optional)

--no-grab

Do no request exclusive access to the input device.

(optional)

vhost-user-gpu 

Command line options:

--render-node=PATH

Specify the GPU DRM render node.

(optional)

--virgl

Enable virgl rendering support.

(optional)

vhost-user-blk 

Command line options:

--blk-file=PATH

Specify block device or file path.

(optional)

--read-only

Enable read-only.

(optional)