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

[Jason Wang <jasowang@redhat.com>]

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.

Thanks


>
> Thanks,