Vhost-user Protocol ¶

Table of Contents

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, master and slave. Master is the application that shares its virtqueues, in our case QEMU. Slave is the consumer of the virtqueues.

In the current implementation QEMU is the master, and the slave 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 backend processing read & write to a virtual disk. In order to facilitate interoperability between various backend implementations, it is recommended to follow the Backend program conventions.

Master and slave 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 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 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 master messages used for writeable fields - 1: Vhost master 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 master sending message requests and slave 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)

Starting and stopping rings ¶

Client must only process each ring when it is started.

Client must only pass data between the ring and the backend, when the ring is enabled.

If ring is started but disabled, client must process the ring without talking to the backend.

For example, for a networking device, in the disabled state client must not supply any new RX packets, but must process and discard any TX packets.

If VHOST_USER_F_PROTOCOL_FEATURES has not been negotiated, the ring is initialized in an enabled state.

If VHOST_USER_F_PROTOCOL_FEATURES has been negotiated, the ring is initialized in a disabled state. Client must not pass data to/from the backend until ring is enabled by VHOST_USER_SET_VRING_ENABLE with parameter 1, or after it has been disabled by VHOST_USER_SET_VRING_ENABLE with parameter 0.

Each ring is initialized in a stopped state, client must not process it until ring is started, or after it has been stopped.

Client 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.

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

Multiple queue support ¶

Many devices have a fixed number of virtqueues. In this case the master already knows the number of available virtqueues without communicating with the slave.

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

Multiple queue support allows the slave to advertise the maximum number of queues. This is treated as a protocol extension, hence the slave 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 slave supports can be queried with message VHOST_USER_GET_QUEUE_NUM. Master should stop when the number of requested queues is bigger than that.

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

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

Slaves 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.

Masters 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 master may need to track the modifications the slave makes to the memory mapped regions. The client 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, server 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 slave 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 master 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 slave 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 slave opens a ‘userfault’-fd and registers the memory with it; this fd is then passed back over to the master. The master 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 slave. The client indicates support for this via the VHOST_USER_PROTOCOL_F_PAGEFAULT feature.

Memory access ¶

The master sends a list of vhost memory regions to the slave 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 master sends IOTLB entries update & invalidation by sending VHOST_USER_IOTLB_MSG requests to the slave 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 slave is expected to reply with a zero payload, non-zero otherwise.

The slave relies on the slave communication channel (see Slave communication section below) to send IOTLB miss and access failure events, by sending VHOST_USER_SLAVE_IOTLB_MSG requests to the master 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 slave may rely on the reply-ack feature, so the master may send a reply when operation is completed if the reply-ack feature is negotiated and slaves requests a reply. For miss events, completed operation means either master sent an update message containing the IOTLB entry containing requested address and permission, or master sent nothing if the IOTLB miss message is invalid (invalid IOVA or permission).

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

Slave communication ¶

An optional communication channel is provided if the slave declares VHOST_USER_PROTOCOL_F_SLAVE_REQ protocol feature, to allow the slave to make requests to the master.

The fd is provided via VHOST_USER_SET_SLAVE_REQ_FD ancillary data.

A slave may then send VHOST_USER_SLAVE_* messages to the master using this fd communication channel.

If VHOST_USER_PROTOCOL_F_SLAVE_SEND_FD protocol feature is negotiated, slave can send file descriptors (at most 8 descriptors in each message) to master via ancillary data using this fd communication channel.

Inflight I/O tracking ¶

To support reconnecting after restart or crash, slave 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, slave need to allocate an extra buffer to store this information of inflight descriptors and share it with master for persistent. VHOST_USER_GET_INFLIGHT_FD and VHOST_USER_SET_INFLIGHT_FD are used to transfer this buffer between master and slave. And the format of this buffer is described below:

queue0 region

queue1 region

…

queueN region

N is the number of available virtqueues. Slave 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. Slave 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. Slave 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 slave 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_SLAVE_REQ and VHOST_USER_PROTOCOL_F_REPLY_ACK, the former is necessary for getting a message channel from the slave to the master, 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 slave 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

Master message types ¶

VHOST_USER_GET_FEATURES

id: 1
equivalent ioctl: VHOST_GET_FEATURES
master payload: N/A
slave payload: u64

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

VHOST_USER_SET_FEATURES

id: 2
equivalent ioctl: VHOST_SET_FEATURES
master payload: u64

Enable features in the underlying vhost implementation using a bitmask. Feature bit VHOST_USER_F_PROTOCOL_FEATURES signals slave 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
master payload: N/A
slave 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.

Note

Slave that reported 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
master payload: u64

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.

Note

Slave that reported 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
master payload: N/A

Issued when a new connection is established. It sets the current master as an owner of the session. This can be used on the slave as a “session start” flag.

VHOST_USER_RESET_OWNER

id: 4
master payload: N/A

Deprecated

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

VHOST_USER_SET_MEM_TABLE

id: 5
equivalent ioctl: VHOST_SET_MEM_TABLE
master payload: memory regions description
slave payload: (postcopy only) memory regions description

Sets the memory map regions on the slave 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 master. The slave 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
master payload: u64
slave payload: N/A

Sets logging shared memory space.

When slave 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
master 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
master payload: vring state description

Set the size of the queue.

VHOST_USER_SET_VRING_ADDR

id: 9
equivalent ioctl: VHOST_SET_VRING_ADDR
master payload: vring address description
slave 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
master payload: vring state description

Sets the base offset in the available vring.

VHOST_USER_GET_VRING_BASE

id: 11
equivalent ioctl: VHOST_USER_GET_VRING_BASE
master payload: vring state description
slave payload: vring state description

Get the available vring base offset.

VHOST_USER_SET_VRING_KICK

id: 12
equivalent ioctl: VHOST_SET_VRING_KICK
master payload: u64

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
master payload: u64

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
master payload: u64

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
master payload: N/A
slave payload: u64

Query how many queues the backend 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
master payload: vring state description

Signal slave 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
master payload: u64

Ask vhost user backend 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 backend to construct and broadcast the fake RARP.

VHOST_USER_NET_SET_MTU

id: 20
equivalent ioctl: N/A
master payload: u64

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, slave 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
master payload: N/A

Set the socket file descriptor for slave 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, slave 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)
master payload: struct vhost_iotlb_msg
slave payload: u64

Send IOTLB messages with struct vhost_iotlb_msg as payload.

Master sends such requests to update and invalidate entries in the device IOTLB. The slave 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
master payload: vring state description

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 master starts the VQ).

VHOST_USER_GET_CONFIG

id: 24
equivalent ioctl: N/A
master payload: virtio device config space
slave payload: virtio device config space

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

VHOST_USER_SET_CONFIG

id: 25
equivalent ioctl: N/A
master payload: virtio device config space
slave payload: N/A

When VHOST_USER_PROTOCOL_F_CONFIG is negotiated, this message is submitted by the vhost-user master when the Guest changes the virtio device configuration space and also can be used for live migration on the destination host. The vhost-user slave must check the flags field, and slaves 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
master payload: crypto session description
slave payload: crypto session description

Create a session for crypto operation. The server side 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
master payload: u64

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
master payload: N/A
slave payload: userfault fd

When VHOST_USER_PROTOCOL_F_PAGEFAULT is supported, the master advises slave that a migration with postcopy enabled is underway, the slave 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
master payload: N/A

Master advises slave that a transition to postcopy mode has happened. The slave 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
slave payload: u64

Master advises that postcopy migration has now completed. The slave must disable the userfaultfd. The response 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
master payload: inflight description

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

VHOST_USER_SET_INFLIGHT_FD

id: 32
equivalent ioctl: N/A
master payload: inflight description

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

VHOST_USER_GPU_SET_SOCKET

id: 33
equivalent ioctl: N/A
master payload: N/A

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

VHOST_USER_RESET_DEVICE

id: 34
equivalent ioctl: N/A
master payload: N/A
slave payload: N/A

Ask the vhost user backend to disable all rings and reset all internal device state to the initial state, ready to be reinitialized. The backend 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 backend.

VHOST_USER_VRING_KICK

id: 35
equivalent ioctl: N/A
slave payload: vring state description
master payload: N/A

When the VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS protocol feature has been successfully negotiated, this message may be submitted by the master to indicate that a buffer was added to the vring instead of signalling it using the vring’s kick file descriptor or having the slave 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
slave payload: u64

When the VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS protocol feature has been successfully negotiated, this message is submitted by master to the slave. The slave 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 backend 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
slave 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 master to the slave. The message payload contains a memory region descriptor struct, describing a region of guest memory which the slave 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 slave device.

VHOST_USER_REM_MEM_REG

id: 38
equivalent ioctl: N/A
slave 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 master to the slave. The message payload contains a memory region descriptor struct, describing a region of guest memory which the slave 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 slave device.

VHOST_USER_SET_STATUS

id: 39
equivalent ioctl: VHOST_VDPA_SET_STATUS
slave payload: N/A
master payload: u64

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

VHOST_USER_GET_STATUS

id: 40
equivalent ioctl: VHOST_VDPA_GET_STATUS
slave payload: u64
master payload: N/A

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

Slave message types ¶

VHOST_USER_SLAVE_IOTLB_MSG

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

Send IOTLB messages with struct vhost_iotlb_msg as payload. Slave sends such requests to notify of an IOTLB miss, or an IOTLB access failure. If VHOST_USER_PROTOCOL_F_REPLY_ACK is negotiated, and slave set the VHOST_USER_NEED_REPLY flag, master 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
slave payload: N/A
master payload: N/A

When VHOST_USER_PROTOCOL_F_CONFIG is negotiated, vhost-user slave 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 slave to get the latest content. If VHOST_USER_PROTOCOL_F_REPLY_ACK is negotiated, and slave set the VHOST_USER_NEED_REPLY flag, master 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
slave payload: vring area description
master 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. Slave 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
slave payload: vring state description
master payload: N/A

When the VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS protocol feature has been successfully negotiated, this message may be submitted by the slave to indicate that a buffer was used from the vring instead of signalling this using the vring’s call file descriptor or having the master 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
slave payload: vring state description
master payload: N/A

When the VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS protocol feature has been successfully negotiated, this message may be submitted by the slave to indicate that an error occurred on the specific vring, instead of signalling the error file descriptor set by the master 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 client 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 client 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 response 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 client, 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 backends 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 backend programs and facilitate interoperability.

Each backend 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 backend type, and binary location. In addition, it defines rules for management apps for picking the highest priority backend when multiple match the search criteria (see @VhostUserBackend documentation in the schema file).

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

The backend 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 backend 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 backend program is started with the vhost-user socket as file descriptor FDNUM. It is incompatible with –socket-path.
--print-capabilities: Output to stdout the backend capabilities in JSON format, and then exit successfully. Other options and arguments should be ignored, and the backend 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)