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

[Jason Wang]

On 2018/12/12 上午10:48, Yongji Xie wrote:
> 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.


You can get this with the help of the both used ring and last_avail_idx 
I believe. Or maybe you can give us an example?

Thanks


> Thanks,
> Yongji