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Subject: RE: [virtio-dev] [PATCH v2] virtio: Improve queue_reset polarity to match to default reset state
> From: Jason Wang <jasowang@redhat.com> > Sent: Wednesday, April 27, 2022 11:15 PM > > On Wed, Apr 27, 2022 at 11:39 PM Parav Pandit <parav@nvidia.com> wrote: > > > > > > > From: Michael S. Tsirkin <mst@redhat.com> > > > Sent: Wednesday, April 27, 2022 11:30 AM > > > > > > On Wed, Apr 27, 2022 at 02:51:36PM +0000, Parav Pandit wrote: > > > > > > > > > From: Jason Wang <jasowang@redhat.com> > > > > > Sent: Wednesday, April 27, 2022 7:30 AM > > > > > > > > > > On Wed, Apr 27, 2022 at 6:26 PM Parav Pandit <parav@nvidia.com> > wrote: > > > > > > > > > > > > example flow: > > > > > > a) 0,0 -> device init time value > > > > > > b) 1,0 -> vq is enabled by driver and working > > > > > > > > > > Did you see my reply in V1? What's the reason for using write to > > > > > clear behavior that is different from the device status? > > > > > > > > > > We can simply make this as 1, 1 here and let the driver write to > > > > > 0 to reset the virtqueue. > > > > > > > > > > And if we do this, the queue_enable and queue_reset are always > > > > > the same, then we can simply reuse queue_enable. > > > > > > > > > Yes, I know we can make this work using new feature bit + single > > > queue_enable register. > > > > I replied that in v0 to Michael. > > > > > > A bigger question in my eyes is that down the road we might want to > > > be able to stop the ring without having it lose state. > > > The natural interface for that seems to be writing 0 to queue enable. > > Why queue_enable and not queue_reset? > > > > to me this interface is unlikely performant and useful for such case. > > When we want to pause/stop the VQ and query the state we need > performant scheme, that can even work in a batch for all the VQs. > > At that point programming 64 registers to pause/stop VQ without losing > state and querying its indices etc won't be scalable with register interface. > > The register interface to sync indices has already been implemented in real > hardware for years. > Sure. I meant to that when we want to pause a VQ and restart later use-case will require more plumbing than just enable/disable register. And to do that, a register interface won't be performant/scalable. So for the wider use case this may not be the good choice. And I explained the other reason that we lose the state information with this busy-wait register in the other reply to V2 and summary in the github issue too on Michael's request. > > I imagine a AQ (likely) or some other interface. > > So did the queue_enable registers, we need to write 1 to queue_enable for > each virtqueue before DRIVER_OK. > > Where to allow writing 0 to queue_enable is orthogonal to scalability. Sure, lets sync after you get chance to go through my other reply to Michael about why with single busy-wait register we lose the state. And hence, queue_reset register (with the fix) is better.
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