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

[Jason Wang <jasowang@redhat.com>]

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):
>      This indicates compliance with Virtio version 1.2 specification.
>
> VIRTIO_F_RING_SPLIT(40):
>      This feature indicates support for the split virtqueue layout as
>      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)?

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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