Re: [Qemu-devel] [PATCH v4 for-4.0 4/7] libvhost-user: Support tracking inflight I/O in shared memory

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

On Fri, Jan 18, 2019 at 11:32:03AM +0800, Yongji Xie wrote:
> On Thu, 17 Jan 2019 at 17:57, Jason Wang <jasowang@redhat.com> wrote:
> >
> >
> > On 2019/1/15 下午10:51, Yongji Xie wrote:
> > >> Well, this may work but here're my points:
> > >>
> > >> 1) The code want to recover from backed crash by introducing extra space
> > >> to store inflight data, but it still depends on the backend to set/get
> > >> the inflight state
> > >>
> > >> 2) Since the backend could be killed at any time, the backend must have
> > >> the ability to recover from the partial inflight state
> > >>
> > >> So it looks to me 1) tends to be self-contradictory and 2) tends to be
> > >> recursive. The above lines show how tricky could the code looks like.
> > >>
> > >> Solving this at vhost-user level through at backend is probably wrong.
> > >> It's time to consider the support from virtio itself.
> > >>
> > > I agree that supporting this in virtio level may be better. For
> > > example, resubmitting inflight I/O once DEVICE_NEEDS_RESET is set in
> > > Stefan's proposal. But I still think QEMU should be able to provide
> > > this ability too. Supposed that one vhost-user backend need to support
> > > multiple VMs. We can't enable reconnect ability until all VMs' guest
> > > driver support the new feature. It's limited.
> >
> >
> > That's the way virtio evolves.
> >
> >
> > >   But if QEMU have the
> > > ability to store inflight buffer, the backend could at least have a
> > > chance to support this case.
> >
> >
> > The problem is, you need a careful designed protocol described somewhere
> 
> That's what we should discuss in detail in this series.
> 
> > (is vhost-user.txt a good place for this?). And this work will be
> > (partial) duplicated for the future support from virtio spec itself.
> >
> 
> I think the duplicated code is to maintain the inflight descriptor
> list which should be in backend. That's not main work in this series.
> And backend could choose to include it or not.
> 
> Thanks,
> Yongji

It would be if someone volunteered to rewrite the vhost user informal
description that we have in qemu and make it a full spec.
So far a text + implementation in contrib seems plenty to me.

-- 
MST