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

[Jason Wang]

On 2019/1/15 下午11:58, Michael S. Tsirkin wrote:
> On Tue, Jan 15, 2019 at 03:52:21PM +0800, Jason Wang 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.
> This is a well studied field. Basically you make sure you commit with an
> atomic write.  Restartable sequences allow accelerating this even
> further.


I'm not sure I get this. But the issue is to exactly deduce all the 
inflight descriptors even if backend could be killed when doing the 
logging. If we could not be 100% accurate, it's have much less value.


> > Solving this at vhost-user level through at backend is probably wrong. It's
> > time to consider the support from virtio itself.
> >
> > Thanks
> I think both approaches have their space.


But there will be a lot of duplicated work if we decide to support it 
from virtio.

Thanks


> -- MST