Packed ring only devices clarification

[Maxime Coquelin <maxime.coquelin@redhat.com>]

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?

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.


Best regards,
Maxime

[0]:
https://docs.oasis-open.org/virtio/virtio/v1.1/csprd01/virtio-v1.1-csprd01.html#x1-230005