[Qemu-devel] [PATCH RFC 00/14] vhost-user: shutdown and reconnection

[marcandre.lureau@redhat.com]

From: Marc-André Lureau <marcandre.lureau@redhat.com>

In a previous series "Add feature to start QEMU without vhost-user
backend", Tetsuya Mukawa proposed to allow the vhost-user backend to
disconnect and reconnect. However, Michael Tsirkin pointed out that
you can't do that without extra care, because the guest and hypervisor
don't know the slave ring manipulation state, there might be pending
replies for example that could be lost, and suggested to reset the
guest queues, but this requires kernel changes, and it may have to
clear the ring and lose queued packets.

The following series starts from the idea that the slave can request a
"managed" shutdown instead and later recover (I guess the use case for
this is to allow for example to update static dispatching/filter rules
etc)

In order to do it, the slave must be in a good state, that is it
should flush all pending buffers so that resume after
VHOST_SET_VRING_BASE is enough to resume where it lefts. The guest is
made aware of virtio-net disconnection thanks to VIRTIO_NET_S_LINK_UP
status, so communication can be stopped.

Unfortunately, vhost-user protocol isn't bidirectional, so a new
optional communication channel is added for the slave to make request
to the master, such as a the new shutdown request.

I have done some testing with modified vapp and linux 4.2, it seems to
work just fine. But more intensive testing and review are required, as
I am not sure this approach can be made solid enough. Before going
further, I would welcome any comment or testing suggestions!

The series is based on top of pending vhost-user migration series, but
for easier testing you may just use the following git repo:
https://github.com/elmarco/qemu vhost-user-reconnect branch

Marc-André Lureau (12):
  vhost-user: remove useless is_server field
  qemu-char: avoid potential double-free
  qemu-char: remove all msgfds on disconnect
  qemu-char: make tcp_chr_disconnect() reentrant-safe
  vhost-net: keep VIRTIO_NET_F_STATUS for vhost-user
  virtio-net: enable tx notification if up and vhost started
  vhost: add vhost_dev stop callback
  vhost-user: add vhost_user to hold the chr
  qemu-char: add qemu_chr_free()
  vhost-user: add slave-fd support
  vhost-user: add shutdown support
  test: start vhost-user reconnect test

Tetsuya Mukawa (2):
  vhost-user: Add ability to know vhost-user backend disconnection
  qemu-char: Add qemu_chr_disconnect to close a fd accepted by listen fd

 docs/specs/vhost-user.txt |  38 +++++++++++
 hw/net/vhost_net.c        |  14 +++-
 hw/net/virtio-net.c       |   4 ++
 hw/virtio/vhost-user.c    | 120 +++++++++++++++++++++++++++++++--
 include/hw/virtio/vhost.h |   4 ++
 include/sysemu/char.h     |  17 ++++-
 net/vhost-user.c          |  20 +++++-
 qemu-char.c               |  23 ++++++-
 tests/Makefile            |   2 +-
 tests/vhost-user-test.c   | 168 ++++++++++++++++++++++++++++++++++++++++++----
 10 files changed, 384 insertions(+), 26 deletions(-)

-- 
2.4.3