From: Prerna Saxena <[email protected]>
The current vhost-user protocol requires the client to send responses to only
few commands. For the remaining commands, it is impossible for QEMU to know the
status of the requested operation -- ie, did it succeed at all, and if so, at
what time.
This is inconvenient, and can also lead to races. As an example:
(1) qemu sends a SET_MEM_TABLE to the backend (eg, a vhost-user net
application) and SET_MEM_TABLE doesn't require a reply according to the spec.
(2) qemu commits the memory to the guest.
(3) guest issues an I/O operation over a new memory region which was configured
on (1)
(4) The application hasn't yet remapped the memory, but it sees the I/O request.
(5) The application cannot satisfy the request because it doesn't know about
those GPAs
Note that the kernel implementation does not suffer from this limitation since
messages are sent via an ioctl(). The ioctl() blocks until the backend (eg.
vhost-net) completes the command and returns (with an error code).
Changing the behaviour of current vhost-user commands would break existing
applications. This patch introduces a protocol extension,
VHOST_USER_PROTOCOL_F_REPLY_ACK. This feature, if negotiated, allows QEMU to
annotate messages to the application that it seeks a response for. The
application must then respond to qemu by providing a status about the requested
operation.
Prerna Saxena (1):
vhost-user : Introduce a new feature, VHOST_USER_PROTOCOL_F_REPLY_ACK
This feature, if negotiated, forces the remote vhost-user
process to send a u64 reply containin status code for each
requested operation.
Status codes are '0' for success, and non-zero for error.
docs/specs/vhost-user.txt | 36 +++++++++++
hw/virtio/vhost-user.c | 153 +++++++++++++++++++++++++++++++++++++++++++++-
2 files changed, 186 insertions(+), 3 deletions(-)
--
1.8.1.2
