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Diffstat (limited to 'docs/specs/vhost-user.txt')
| -rw-r--r-- | docs/specs/vhost-user.txt | 26 |
1 files changed, 26 insertions, 0 deletions
diff --git a/docs/specs/vhost-user.txt b/docs/specs/vhost-user.txt index 777c49cfe6..7890d71698 100644 --- a/docs/specs/vhost-user.txt +++ b/docs/specs/vhost-user.txt @@ -37,6 +37,8 @@ consists of 3 header fields and a payload: * 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 @@ -126,6 +128,8 @@ the ones that do: * VHOST_GET_VRING_BASE * VHOST_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: @@ -254,6 +258,7 @@ 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 Message types ------------- @@ -464,3 +469,24 @@ Message types 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_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.) |