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Subject: Re: [virtio-dev] [PATCH v2] virtio: Improve queue_reset polarity to match to default reset state
On Wed, Apr 27, 2022 at 03:57:35PM +0000, Parav Pandit wrote: > > > From: Michael S. Tsirkin <mst@redhat.com> > > Sent: Wednesday, April 27, 2022 11:44 AM > > > > On Wed, Apr 27, 2022 at 03:39:40PM +0000, Parav Pandit 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? > > > > If what to disable ring without reset then writing into reset seems > > unintuitive. > > > True. I assume you want to start the queue again later, hence the stop/start. > Make sense. > > > > 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. > > > I imagine a AQ (likely) or some other interface. > > > > > > > > > > > > I was not sure how drastic that would be at this point in the spec > > > > > release cycle > > > > that Michael highlighted. > > > > > Hence, I proposed a minimal change fix to queue_reset register given > > timeline. > > > > > > > > Well if accepted this proposal is going to delay the release anyway. > > > > If we are doing a new feature then that can love alongside the one > > > > that is already in the spec. > > > I didn't quite understand your point. > > > > I understood your "given timeline" to mean "to avoid delays in 1.2 release". > Yes. > > > My point is any material change will mean a delay at this time. > But this is so basic. > It's hard to gaze at this spec for coming years and the code to see, > Hey sometimes 0 means disabled, sometime 0 means still enabled, sometime 1 means enabled, and sometimes 1 means now disabled... > And maintain those weird code in device side and extra state bits burning some expensive chip resource. > Is removing from 1.2 is equal delay to get is fixed in 1.3? > If yes, I make humble request to fix this and have errata. > Some of the professional standard bodies release the spec and short after that errata/ratification follows the release that resolve such small issues. > May be time for virtio spec to take this opportunity now and be bit agile on it. > Your call. :) I would suggest waiting for results of public review. The TC can then decide whether to release spec as is and then another version with bug fixes, or delay this one. -- MST
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