Re: [virtio-dev] Packed ring only devices clarification

[Jason Wang <jasowang@redhat.com>]

On 2021/1/29 äå5:14, Maxime Coquelin wrote:
> On 1/29/21 10:04 AM, Jason Wang wrote:
> > On 2021/1/29 äå4:38, Maxime Coquelin wrote:
> > > Hello,
> > >
> > > In the Virtqueues section of the Virtio specification [0], it is
> > > mentioned that:
> > >
> > > "
> > > Every driver and device supports either the Packed or the Split
> > > Virtqueue format, or both.
> > > "
> > >
> > > However, it seems there is no way to differentiate whether the device
> > > supports both split and packed ring, or only packed ring by reading
> > > its features bits.
> > >
> > > In the case of live-migration, it means we would need to trigger a
> > > features negotiation before the migration is initiated, and see if
> > > FEATURES_OK device status bit is set. Still, we could not be sure a
> > > failure would be due to the device supporting only packed ring.
> > >
> > > Is my understanding correct?
> >
> > Yes, I think so.
> >
> >
> > > If so, maybe we would need to introduce a new feature bit for split
> > > ring, and also a feature bit indicating compliance with the new spec
> > > version for backward compatibility:
> > >
> > > VIRTIO_F_VERSION_1_2(39):
> > >  ÂÂÂÂ This indicates compliance with Virtio version 1.2 specification.
> > >
> > > VIRTIO_F_RING_SPLIT(40):
> > >  ÂÂÂÂ This feature indicates support for the split virtqueue layout as
> > >  ÂÂÂÂ described in 2.6 Split Virtqueues.
> >
> > A question here, do we want to fix this issues with old driver (when
> > both of the above are not negotiated)?
> Could you elaborate please? I am not sure to understand your question.


I meant it looks to me we can't avoid the feature pre-negotiation, if we 
migrate to an elder qemu?


> For existing devices only supporting packed ring (if it exists, I don't
> know any for now),


Spec doesn't forbid that so I guess there would be some hardware 
implementation will do packed only.


>   maybe their drivers could have a quirk adding above
> features.


It won't be easy consider there will be a software updating in the guest.


>   If we do that, we may want to rename VIRTIO_F_VERSION_1_2 with
> something else to restrict it to rings layout support.


This looks like a common issue which is not packed specific, it might 
happen when destination mandate some features but src doesn't (e.g 
VIRTIO_F_ACCESS_PLATFORM).

Maybe it's the time to introduce an API in the spec to know the features 
that is forced by the device.

Thanks


> Thanks,
> Maxime
>
> > Thanks
> >
> >
> > > Best regards,
> > > Maxime
> > >
> > > [0]:
> > > https://docs.oasis-open.org/virtio/virtio/v1.1/csprd01/virtio-v1.1-csprd01.html#x1-230005
> > >
> > >
> > >
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