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 09:57:22PM +0800, Jason Wang wrote:
> 
> On 2018/12/6 下午2:35, 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.
> 
> 
> I wonder whether or not it's better to handle this at the level of virtio
> protocol itself instead of vhost-user level. E.g expose last_avail_idx to
> driver might be sufficient?
> 
> Another possible issue is, looks like you need to deal with different kinds
> of ring layouts e.g packed virtqueues.
> 
> Thanks

I'm not sure I understand your comments here.
All these would be guest-visible extensions.
Possible for sure but how is this related to
a patch supporting transparent reconnects?

> 
> > 
> > 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
> > 
> >   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(-)
> >