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[Qemu-devel] [PATCH for-2.7 v5.1 0/2] vhost-user: Extend protocol to rec

From: Prerna Saxena
Subject: [Qemu-devel] [PATCH for-2.7 v5.1 0/2] vhost-user: Extend protocol to receive replies on any command.
Date: Thu, 28 Jul 2016 00:07:21 -0700

From: Prerna Saxena <address@hidden>

vhost-user: Extend protocol to receive replies on any command.

The current vhost-user protocol requires the client to send reply 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 
(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 
Patch 1 introduces a protocol extension, VHOST_USER_PROTOCOL_F_REPLY_ACK. This
feature, if negotiated, allows QEMU to request a reply to any message by setting
the newly introduced "need_reply" flag. The application must then respond to 
by providing a status about the requested operation.

Patch 2 adds a workaround for the race described above for clients that do not 
support REPLY_ACK
feature. It introduces  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 messagesin order.

Changes v5->v5.1 :
1) Patch 1 : no change
2) Patch 2 : fixes a tiny typo I'd accidentally introduced while creating v5 
from v4. The code itself is unchanged from v4.

Changes v4->v5:
1) Patch 1 :
* Reword 'response' to 'reply' on public demand.
* Documentation is more concise.
Patch 2 : unchanged

Changes v3->v4:
1) Rearranged code in PATCH 1 to offset compiler warnings about missing 
declaration of vhost_user_read(). Fixed by moving process_message_reply() after 
definition of vhost_user_read()
2) Fixed minor suggestions in writeup for this protocol extension.

Changes v2->v3:
1) Swapped the patch numbers 1 & 2 from the previous series.
2) Patch 1 (previously patch 2 in v2): addresses MarcAndre's review comments 
and renames function 'process_message_response' to 'process_message_reply'
3) Patch 2 (ie patch 1 in v2) : Unchanged from v2.

Changes v1->v2:
1) Patch 1 : Ask for get_features before returning from set_mem_table(new).
2) 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.

v1 : https://lists.gnu.org/archive/html/qemu-devel/2016-06/msg07152.html
v2 : https://lists.gnu.org/archive/html/qemu-devel/2016-07/msg00048.html
v3 : https://lists.gnu.org/archive/html/qemu-devel/2016-07/msg01598.html
v4 : https://lists.gnu.org/archive/html/qemu-devel/2016-07/msg06173.html

Prerna Saxena (2):
  vhost-user: Introduce a new protocol feature REPLY_ACK.
  vhost-user: Attempt to fix a race with set_mem_table.

 docs/specs/vhost-user.txt |  44 +++++++++++++++
 hw/virtio/vhost-user.c    | 133 ++++++++++++++++++++++++++++++----------------
 2 files changed, 130 insertions(+), 47 deletions(-)


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