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

[Jason Wang <jasowang@redhat.com>]

On 2018/12/12 下午5:18, Yongji Xie wrote:
>>>> Ok, then we can simply forbid increasing the avail_idx in this case?
>>>>
>>>> Basically, it's a question of whether or not it's better to done it in
>>>> the level of virtio instead of vhost. I'm pretty sure if we expose
>>>> sufficient information, it could be done without touching vhost-user.
>>>> And we won't deal with e.g migration and other cases.
>>>>
>>> OK, I get your point. That's indeed an alternative way. But this feature seems
>>> to be only useful to vhost-user backend.
>> I admit I could not think of a use case other than vhost-user.
>>
>>
>>>    I'm not sure whether it make sense to
>>> touch virtio protocol for this feature.
>> Some possible advantages:
>>
>> - Feature could be determined and noticed by user or management layer.
>>
>> - There's no need to invent ring layout specific protocol to record in
>> flight descriptors. E.g if my understanding is correct, for this series
>> and for the example above, it still can not work for packed virtqueue
>> since descriptor id is not sufficient (descriptor could be overwritten
>> by used one). You probably need to have a (partial) copy of descriptor
>> ring for this.
>>
>> - No need to deal with migration, all information was in guest memory.
>>
> Yes, we have those advantages. But seems like handle this in vhost-user
> level could be easier to be maintained in production environment. We can
> support old guest. And the bug fix will not depend on guest kernel updating.


Yes. But the my main concern is the layout specific data structure. If 
it could be done through a generic structure (can it?), it would be 
fine. Otherwise, I believe we don't want another negotiation about what 
kind of layout that backend support for reconnect.

Thanks


>
> Thanks,
> Yongji