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Diffstat (limited to 'src/docs/specs/vhost-user.txt')
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diff --git a/src/docs/specs/vhost-user.txt b/src/docs/specs/vhost-user.txt new file mode 100644 index 0000000..0312d40 --- /dev/null +++ b/src/docs/specs/vhost-user.txt @@ -0,0 +1,466 @@ +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 + * 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 user address of the vring descriptor table + Used: a 64-bit user address of the vring used ring + Available: a 64-bit user address of the vring available ring + Log: a 64-bit guest address for logging + + * 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) + +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; + }; +} 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_GET_FEATURES + * VHOST_GET_PROTOCOL_FEATURES + * VHOST_GET_VRING_BASE + * VHOST_SET_LOG_BASE (if VHOST_USER_PROTOCOL_F_LOG_SHMFD) + +There are several messages that the master sends with file descriptors passed +in the ancillary data: + + * VHOST_SET_MEM_TABLE + * VHOST_SET_LOG_BASE (if VHOST_USER_PROTOCOL_F_LOG_SHMFD) + * VHOST_SET_LOG_FD + * VHOST_SET_VRING_KICK + * VHOST_SET_VRING_CALL + * VHOST_SET_VRING_ERR + +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 queues the slave supports can be queried with message +VHOST_USER_GET_PROTOCOL_FEATURES. 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. + +Protocol features +----------------- + +#define VHOST_USER_PROTOCOL_F_MQ 0 +#define VHOST_USER_PROTOCOL_F_LOG_SHMFD 1 +#define VHOST_USER_PROTOCOL_F_RARP 2 + +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 + + 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. + + * 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 + + Sets the number of vrings for this owner. + + * 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 quried 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. |
