Re: [virtio-dev] queue_reset register polarity to improve

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

On Sun, Apr 24, 2022 at 03:28:53PM +0800, Xuan Zhuo wrote:
> On Sun, 24 Apr 2022 00:49:19 +0000, Parav Pandit <parav@nvidia.com> wrote:
> > Hi,
> >
> > A recently defined queue_reset register has a little weird definition that we should improve.
> > When driver initiate queue reset, it writes queue_reset = 1.
> > When device is busy resetting the queue, on this driver request, it is expected to return queue_reset=0.
> > Once queue reset is completed it is expected to return queue_reset = 1.
> > (Polarity changed twice to same value as what was driver set). See more below.
> >
> > So state wise,
> > # q_enable, q_reset represents :
> > a) 0,0 -> device init time value
> > b) 1,0 -> vq is enabled and working
> > c) 1,1 -> vq is enabled, driver initiated reset
> > d) 1,0 -> vq is enabled, but device is busy doing the reset (conflicting definition with above #b )
> > e) 0,1 -> vq reset is complete in the device and VQ is now disabled (again conflict with #a above )
> >
> > Instead, I think we should have below better, consistent definition, no matter how queue reset occurs (init time or later).
> >
> > q_enable, q_reset
> > A) 0, 0 -> default, device init time
> > B) 1, 0 -> driver has enabled vq
> > C) 1, 1 -> driver started q reset
> > D) 1, 1 -> q_reset stays 1 until device is busy resetting vq (communicating that its working on resetting, consistent with #C)
> > E) 0, 0 -> q_reset by device is completed, q got disabled (now matches the state same as device init time #A)
> 
> Seems to me to be two different designs, I don't see any particular value in the
> conflict mentioned here, and under what circumstances would it cause any
> trouble?
> 
> The design here is similar to many scenarios, such as device reset.

Hmm. with device reset we have a reverse polarity:

	The driver SHOULD consider a driver-initiated reset complete when it
	reads \field{device status} as 0.

in what sense is the current design similar?

> So if you want to change to your design, I want to know if there are other
> reasons.
> 
> Thanks.
> 
> >
> > Parav
> >
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