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

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

On Fri, Dec 14, 2018 at 09:56:41AM +0800, Yongji Xie wrote:
> On Thu, 13 Dec 2018 at 22:45, Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > 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?
> >
> 
> Definitely, yes. And the comments looks good to me. Qemu get size and
> version from backend, then allocate memory and send it back with
> version. Backend knows how to use the memory according to the version.
> If we do that, should we allocate the memory per device rather than
> per virtqueue?
> 
> Thanks,
> Yongji

It's up to you. Maybe both.

-- 
MST