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[Qemu-devel] [PATCH 0/3] vhost-user reconnect

From: Yury Kotov
Subject: [Qemu-devel] [PATCH 0/3] vhost-user reconnect
Date: Thu, 16 Aug 2018 18:32:40 +0300

We are using QEMU (2.12.0) with SPDK (18.04.1) over vhost-user to emulate block
devices. One of our cases it to restart SPDK without restarting VM (in case
of some updates or smth like it). We tried to use the 'reconnect' option for
the '-chardev' device:
  -object memory-backend-file,id=mem0,size=1G,mem-path=/dev/hugepages,share=on \
  -numa node,memdev=mem0 \
  -chardev socket,id=spdk_vhost_blk1,path=/var/tmp/vhost.1,reconnect=10 \
  -device vhost-user-blk-pci,chardev=spdk_vhost_blk1,num-queues=4

After this, vhost-user-blk initialization fails with an error below:
  qemu-system-x86_64: -device ...: Failed to set msg fds.
  qemu-system-x86_64: -device ...: vhost-user-blk: vhost initialization failed:
                                   Operation not permitted

We got the same error with the latest QEMU (c542a9f9794ec8e0bc3f).

We made some investigations and found out that there are several issues:

1. Reconnect option postpones the first connection till machine init done event.
   But we need this connection during vhost blk device initialization which
   happens before the machine init done handling.

2. If the connection is forced, then the reconnection will be successful
   after SPDK restart. The problem is that virtual queue will not start.
   The reason for it is that virtual queue initialization commands
   should be resent:

The patch set resolves both of these issues.

Test case:

1. Start fio process (inside VM):
     fio --name test --ioengine=libaio --iodepth=64 --bs=4096 \
         --rw=randrw --direct=1 --sync=1 --verify=md5 \
         --size=64M --filename=/dev/vda --loops=100

2. Restart SPDK many times.
   We are expecting that during SPDK restart fio will pause and fio should
   continue to work after restart completion.

3. fio process completed successfully without any error.

Yury Kotov (3):
  chardev: prevent extra connection attempt in tcp_chr_machine_done_hook
  vhost: refactor vhost_dev_start and vhost_virtqueue_start
  vhost-user: add reconnect support for vhost-user

 chardev/char-socket.c     |   5 +-
 hw/virtio/vhost-user.c    |  65 ++++++++++++--
 hw/virtio/vhost.c         | 223 +++++++++++++++++++++++++++++++---------------
 include/hw/virtio/vhost.h |   2 +
 4 files changed, 215 insertions(+), 80 deletions(-)


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