Vhost-user Protocol =================== Copyright (c) 2014 Virtual Open Systems Sarl. This work is licensed under the terms of the GNU GPL, version 2 or later. See the COPYING file in the top-level directory. =================== 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 intended to be a software Ethernet switch running in user space, such as Snabbswitch. Master and slave can be either a client (i.e. connecting) or server (listening) in the socket communication. Message Specification --------------------- Note that 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 | ------------------------------------ * 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 slave - Bit 3 is the need_reply flag - see VHOST_USER_PROTOCOL_F_REPLY_ACK for details. * Size - 32-bit size of the 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 * 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: a 8-bit value: - 0: No access - 1: Read access - 2: Write access - 3: Read/Write access Type: a 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 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; }; } 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) [ Also see the section on REPLY_ACK protocol extension. ] There are several messages that the master sends with file descriptors passed in the ancillary data: * 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 If Master is unable to send the full message or receives a wrong reply it will close the connection. An optional reconnection mechanism can be implemented. Any protocol extensions are gated by protocol feature bits, which allows full backwards compatibility on both master and slave. As older slaves don't support negotiating protocol features, a feature bit was dedicated for this purpose: #define VHOST_USER_F_PROTOCOL_FEATURES 30 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, 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 ---------------------- Multiple queue 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 queue pairs 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. One queue pair is enabled initially. More queues are enabled dynamically, by sending message VHOST_USER_SET_VRING_ENABLE. 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 communcation 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. 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 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 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 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 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 acknowledgment; 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 a kick. * 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. * 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. * 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 endianess 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 endianesses 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. 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_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.)