# 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

### Header

request:

  32-bit type of the request

flags:

  32-bit bit field

- Lower 2 bits are the version (currently 0x01)

- Bit 2 is the reply flag - needs to be sent on each reply from the back-end

- Bit 3 is the need_reply flag - see REPLY_ACK for details.

size:

  32-bit size of the 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 address description

index | flags | 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 regions description

num regions | padding | region0 | … | region7

num regions:

  a 32-bit number of regions

padding:

  32-bit

A region is:

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

#### Single memory region description

padding | guest address | size | user address | mmap offset

padding:

  64-bit

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

#### 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

### 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`)

See also

REPLY_ACK

  The section on `REPLY_ACK` protocol extension.

There are several messages that the front-end sends with file descriptors passed in the ancillary data:

- `VHOST_USER_ADD_MEM_REG`

- `VHOST_USER_SET_MEM_TABLE`

- `VHOST_USER_SET_LOG_BASE` (if `VHOST_USER_PROTOCOL_F_LOG_SHMFD`)

- `VHOST_USER_SET_LOG_FD`

- `VHOST_USER_SET_VRING_KICK`

- `VHOST_USER_SET_VRING_CALL`

- `VHOST_USER_SET_VRING_ERR`

- `VHOST_USER_SET_SLAVE_REQ_FD`

- `VHOST_USER_SET_INFLIGHT_FD` (if `VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD`)

If front-end is unable to send the full message or receives a wrong reply it will close the connection. An optional reconnection mechanism can be implemented.

If back-end detects some error such as incompatible features, it may also close the connection. This should only happen in exceptional circumstances.

Any protocol extensions are gated by protocol feature bits, which allows full backwards compatibility on both front-end and back-end. As older back-ends don’t support negotiating protocol features, a feature bit was dedicated for this purpose:

    #define VHOST_USER_F_PROTOCOL_FEATURES 30

Note that VHOST_USER_F_PROTOCOL_FEATURES is the UNUSED (30) feature bit defined in VIRTIO 1.1 6.3 Legacy Interface: Reserved Feature Bits. VIRTIO devices do not advertise this feature bit and therefore VIRTIO drivers cannot negotiate it.

This reserved feature bit was reused by the vhost-user protocol to add vhost-user protocol feature negotiation in a backwards compatible fashion. Old vhost-user front-end and back-end implementations continue to work even though they are not aware of vhost-user protocol feature negotiation.

### Ring states

Rings can be in one of three states:

- stopped: the back-end must not process the ring at all.

- started but 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.

Each ring is initialized in a stopped state. The back-end must start 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 ring upon receiving `VHOST_USER_GET_VRING_BASE`.

Rings can be enabled or disabled by `VHOST_USER_SET_VRING_ENABLE`.

If `VHOST_USER_F_PROTOCOL_FEATURES` has not been negotiated, the ring starts directly in the enabled state.

If `VHOST_USER_F_PROTOCOL_FEATURES` has been negotiated, the ring is initialized in a disabled state and is enabled by `VHOST_USER_SET_VRING_ENABLE` with parameter 1.

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

### 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. 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.

### 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_SLAVE_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_SLAVE_REQ` protocol feature, to allow the back-end to make requests to the front-end.

The fd is provided via `VHOST_USER_SET_SLAVE_REQ_FD` ancillary data.

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

If `VHOST_USER_PROTOCOL_F_SLAVE_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:

1. Get the next available head-descriptor index from available ring, `i`

2. Set `desc[i].counter` to the value of global counter

3. Increase global counter by 1

4. Set `desc[i].inflight` to 1

When supplying used buffers to the driver:

1. Get corresponding used head-descriptor index, i

2. Set `desc[i].next` to `last_batch_head`

3. Set `last_batch_head` to `i`

4. Steps 1,2,3 may be performed repeatedly if batching is possible

5. Increase the `idx` value of used ring by the size of the batch

6. Set the `inflight` field of each `DescStateSplit` entry in the batch to 0

7. Set `used_idx` to the `idx` value of used ring

When reconnecting:

1. 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

2. 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:

1. Get the next available descriptor entry from descriptor ring, `d`

2. 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

3. If `d` is last descriptor, set `desc[old_free_head].last` to `free_head`

4. Increase `desc[old_free_head].num` by 1

5. 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`

6. Set `free_head` to `desc[free_head].next`

7. If `d` is last descriptor, set `old_free_head` to `free_head`

When supplying used buffers to the driver:

1. Get corresponding used head-descriptor entry from descriptor ring, `d`

2. Get corresponding `DescStatePacked` entry, `e`

3. Set `desc[e.last].next` to `free_head`

4. Set `free_head` to the index of `e`

5. Steps 1,2,3,4 may be performed repeatedly if batching is possible

6. Increase `used_idx` by the size of the batch and update `used_wrap_counter` if needed

7. Update `d.flags`

8. Set the `inflight` field of each head `DescStatePacked` entry in the batch to 0

9. Set `old_free_head`, `old_used_idx`, `old_used_wrap_counter` to `free_head`, `used_idx`, `used_wrap_counter`

When reconnecting:

1. 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`

2. 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)

3. Set the `inflight` field of each `DescStatePacked` entry in free list to 0

4. 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_SLAVE_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_SLAVE_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_SLAVE_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

### 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:

    memory regions description

  reply payload:

    (postcopy only) 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 state description

  reply payload:

    N/A

  Sets the base offset in the available vring.

`VHOST_USER_GET_VRING_BASE`
  id:

    11

  equivalent ioctl:

    `VHOST_USER_GET_VRING_BASE`

  request payload:

    vring state description

  reply payload:

    vring state description

  Get the available vring base offset.

`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_SLAVE_REQ` have been negotiated this message isn’t necessary as the `VHOST_USER_SLAVE_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_SLAVE_REQ` have been negotiated this message isn’t necessary as the `VHOST_USER_SLAVE_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_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_SLAVE_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.

### 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_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_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_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_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_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_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)