Re: [Qemu-devel] [PATCH for-4.0 0/6] vhost-user-blk: Add support for backend reconnecting

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Dec 06, 2018 at 02:35:46PM +0800, elohimes@gmail.com wrote:
> From: Xie Yongji <xieyongji@baidu.com>
> 
> This patchset is aimed at supporting qemu to reconnect
> vhost-user-blk backend after vhost-user-blk backend crash or
> restart.
> 
> The patch 1 tries to implenment the sync connection for
> "reconnect socket".
> 
> The patch 2 introduces a new message VHOST_USER_SET_VRING_INFLIGHT
> to support offering shared memory to backend to record
> its inflight I/O.
> 
> The patch 3,4 are the corresponding libvhost-user patches of
> patch 2. Make libvhost-user support VHOST_USER_SET_VRING_INFLIGHT.
> 
> The patch 5 supports vhost-user-blk to reconnect backend when
> connection closed.
> 
> The patch 6 tells qemu that we support reconnecting now.
> 
> To use it, we could start qemu with:
> 
> qemu-system-x86_64 \
>         -chardev socket,id=char0,path=/path/vhost.socket,reconnect=1,wait \
>         -device vhost-user-blk-pci,chardev=char0 \
> 
> and start vhost-user-blk backend with:
> 
> vhost-user-blk -b /path/file -s /path/vhost.socket
> 
> Then we can restart vhost-user-blk at any time during VM running.
> 
> Xie Yongji (6):
>   char-socket: Enable "wait" option for client mode
>   vhost-user: Add shared memory to record inflight I/O
>   libvhost-user: Introduce vu_queue_map_desc()
>   libvhost-user: Support recording inflight I/O in shared memory
>   vhost-user-blk: Add support for reconnecting backend
>   contrib/vhost-user-blk: enable inflight I/O recording

What is missing in all this is documentation.
Specifically docs/interop/vhost-user.txt.

At a high level the design is IMO a good one.

However I would prefer reading the protocol first before
the code.

So here's what I managed to figure out, and it matches
how I imagined it would work when I was still
thinking about out of order for net:

- backend allocates memory to keep its stuff around
- sends it to qemu so it can maintain it
- gets it back on reconnect

format and size etc are all up to the backend,
a good implementation would probably implement some
kind of versioning.

Is this what this implements?

>  chardev/char-socket.c                   |   5 +-
>  contrib/libvhost-user/libvhost-user.c   | 215 ++++++++++++++++++++----
>  contrib/libvhost-user/libvhost-user.h   |  19 +++
>  contrib/vhost-user-blk/vhost-user-blk.c |   3 +-
>  hw/block/vhost-user-blk.c               | 169 +++++++++++++++++--
>  hw/virtio/vhost-user.c                  |  69 ++++++++
>  hw/virtio/vhost.c                       |   8 +
>  include/hw/virtio/vhost-backend.h       |   4 +
>  include/hw/virtio/vhost-user-blk.h      |   4 +
>  include/hw/virtio/vhost-user.h          |   8 +
>  10 files changed, 452 insertions(+), 52 deletions(-)
> 
> -- 
> 2.17.1