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

[Michael S. Tsirkin]

On Thu, Jan 17, 2019 at 05:57:29PM +0800, Jason Wang 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 (is
> vhost-user.txt a good place for this?). And this work will be (partial)
> duplicated for the future support from virtio spec itself.
> 
> 
> > Maybe backend could have other way to
> > avoid the tricky code.
> 
> 
> I'm not sure, but it was probably not easy.

I see an implementation in libvhost-user.

Do you see an issue there?


> Thanks
> 
> 
> > 
> > Thanks,