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

[Jason Wang <jasowang@redhat.com>]

On Sun, Apr 24, 2022 at 2:47 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Sun, Apr 24, 2022 at 12:49:19AM +0000, Parav Pandit 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 )
> it is not great but don't see a conflict here
>
> > e) 0,1 -> vq reset is complete in the device and VQ is now disabled (again conflict with #a above )
> this one is ugly in that state is really mostly same as (1)
> but the flag values are different
>
>
> > f) 1,0 -> vq is enabled and working again
>
>
>
> It can actually be any value, the spec just says
>
> If VIRTIO_F_RING_RESET has been negotiated, after the driver writes 1 to
> \field{QueueReset} to reset the queue, it MUST verify that the queue
> has been reset by reading back \field{QueueReset} and ensuring that it
> is 1.
>
> So can be 2 or whatever, so one can distinguish between the two states.
>
> Spec really should clarify what to do if it is not 1 (i.e. read it again
> until it is 1) .
>
>
> > 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)
> >
> > Parav
>
>
> Well it's been merged since November. Probably too late unless you can
> convince the TC that the current feature should be abandoned
> and the feature completely redesigned. Above does not look like
> a deal breaker.
>
> If we are to re-design it, I would maybe instead rework things so queue_enable can be
> written to, to stop vq without a reset.

So to make it work like MMIO's QueueReady? Btw, it's not clear what
happens if virtqueue can hold its state when write 0 to QueueReady.

Thanks

> Will need careful work for
> transports other than PCI since those already allow writing into e.g.
> QueueReady.
>
> If possible, please open a github issue so we can track this for the
> release.
>
> --
> MST
>
>
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