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

[Yongji Xie]

On Mon, 10 Dec 2018 at 17:32, Jason Wang <address@hidden> wrote:
>
>
> On 2018/12/6 下午9:59, Michael S. Tsirkin wrote:
> > On Thu, Dec 06, 2018 at 09:57:22PM +0800, Jason Wang wrote:
> >> On 2018/12/6 下午2:35,address@hidden  wrote:
> >>> From: Xie Yongji<address@hidden>
> >>>
> >>> 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.
>
>
> Looks not, it only introduces a shared memory between qemu and
> vhost-user backend?
>
>
> > Possible for sure but how is this related to
> > a patch supporting transparent reconnects?
>
>
> I might miss something. My understanding is that we support transparent
> reconnects, but we can't deduce an accurate last_avail_idx and this is
> what capability this series try to add. To me, this series is functional
> equivalent to expose last_avail_idx (or avail_idx_cons) in available
> ring. So the information is inside guest memory, vhost-user backend can
> access it and update it directly. I believe this is some modern NIC did
> as well (but index is in MMIO area of course).
>

Hi Jason,

If my understand is correct, it might be not enough to only expose
last_avail_idx.
Because we would not process descriptors in the same order in which they have
been made available sometimes. If so, we can't get correct inflight
I/O information
from available ring.

Thanks,
Yongji