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[Qemu-devel] [PATCH 0/1] vhost-user: Add a protocol extension for client

From: Prerna Saxena
Subject: [Qemu-devel] [PATCH 0/1] vhost-user: Add a protocol extension for client responses to vhost commands.
Date: Fri, 24 Jun 2016 01:17:07 -0700

From: Prerna Saxena <address@hidden>

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 

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(-)


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