[Qemu-devel] [PATCH v2 0/2]vhost-user: Extend protocol to seek response for any command.

[Prerna Saxena]

From: Prerna Saxena <address@hidden>

The current vhost-user protocol requires the client to send responses to only a
few commands. For the remaining commands, it is impossible for QEMU to know the
status of the requested operation -- ie, did it succeed? If so, by 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).
Note that SET_MEM_TABLE does not 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 does not 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. 
To work around this race, Patch 1 adds a get_features command to be sent before 
returning from set_mem_table. While this is not a complete fix, it will help 
client applications that strictly process messages in order.

The second patch introduces a protocol extension, 
VHOST_USER_PROTOCOL_F_REPLY_ACK. This feature, if negotiated, allows QEMU to 
request a response to any message by setting the newly introduced 
"need_response" flag. The application must then respond to qemu by providing a 
status about the requested operation.

Changelog:
---------
Changes since v1:
Patch 1 : Ask for get_features before returning from set_mem_table(new). 
Patch 2 : * Improve documentation. 
          * Abstract out commonly used operations in the form of a function, 
process_message_response(). Also implement this only for SET_MEM_TABLE.

Prerna Saxena (2):
  vhost-user: Attempt to prevent a race on set_mem_table.
  vhost-user : Introduce a new feature VHOST_USER_PROTOCOL_F_REPLY_ACK.

 docs/specs/vhost-user.txt |  40 ++++++++++++
 hw/virtio/vhost-user.c    | 157 ++++++++++++++++++++++++++++++++--------------
 2 files changed, 150 insertions(+), 47 deletions(-)

-- 
1.8.1.2