Re: [virtio-dev] [PATCH v2] virtio: Improve queue_reset polarity to match to default reset state

["Michael S. Tsirkin" <mst@redhat.com>]

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