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

[Yongji Xie <elohimes@gmail.com>]

On Thu, 6 Dec 2018 at 21:57, Jason Wang <jasowang@redhat.com> 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.
>

Yes, we should be able to deal with the packed virtqueues. But this
should be processed in backend rather than in qemu. Qemu just sends
a shared memory to backend. Then backend use it to record inflight I/O
and do I/O replay when necessary.

Thanks,
Yongji