Re: [Qemu-devel] [PATCH 14/15] hw/block/virtio-blk: create num_queues vqs if dataplane is enabled

[Paolo Bonzini]

Il 31/07/2014 05:47, Ming Lei ha scritto:
> On Wed, Jul 30, 2014 at 11:25 PM, Paolo Bonzini <address@hidden> wrote:
>> Il 30/07/2014 17:12, Michael S. Tsirkin ha scritto:
>>>>>
>>>>> Dataplane must not be a change to the guest ABI.  If you implement this
>>>>> feature you have to implement it for both dataplane and non-dataplne.
>>>>>
> 
> IMO, no matter if the feature is set or not, both old and new VM
> can support it well.
> 
> Per virtio spec, only the feature is set, the VM can be allowed to
> access the 'num_queues' field, and I didn't see any problem from
> VM's view point.
> 
> So could you explain why both dataplane and non-dataplane have
> to support the feature.

Because otherwise you change the guest ABI.

> I am doing so because there isn't performance advantage to take mq
> for non-dataplane.

It doesn't matter, changing features between dataplane and non-dataplane
is not something that you can do.

>>> Actually, I think different backends at runtime should be allowed to
>>> change guest visible ABI.  For example if you give qemu a read only
>>> backend this is guest visible right?
>>
>> Dataplane is not meant to be a different backend, it's just moving stuff
>> out to a separate thread.  It's only due to QEMU limitation that it
>> doesn't share the vring code (and these patches include a lot of steps
>> backwards where dataplane becomes != non-dataplane).
> 
> There won't lots of backwards steps, just the bypass co patch, which
> is just bypassing co in case of being unnecessary for raw image case,
> but it is still one code path.

Using the object pool for dataplane only is a backwards step,
implementing multique for dataplane only is a backwards step, bypassing
coroutines for dataplane only is a backwards step.

Paolo