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 03:39:40PM +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.

> 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". My point is any material change will mean a delay at this
time.


> This queue_reset in its current state (with/without) this proposed fix is mostly usable within the guest for dynamic VQ creation/deletion to connect to ethtool/xdp or more.
> 
> I don't see a need to delay the fix, to a larger feature that needs more than just start/stop button.



-- 
MST