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Subject: Re: [virtio-dev] Packed ring only devices clarification
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. For existing devices only supporting packed ring (if it exists, I don't know any for now), maybe their drivers could have a quirk adding above features. If we do that, we may want to rename VIRTIO_F_VERSION_1_2 with something else to restrict it to rings layout support. Thanks, Maxime > Thanks > > >> >> >> Best regards, >> Maxime >> >> [0]: >> https://docs.oasis-open.org/virtio/virtio/v1.1/csprd01/virtio-v1.1-csprd01.html#x1-230005 >> >> >> >> --------------------------------------------------------------------- >> To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org >> For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org >>
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