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

[Xuan Zhuo <xuanzhuo@linux.alibaba.com>]

On Wed, 27 Apr 2022 19:29:37 +0800, Jason Wang <jasowang@redhat.com> wrote:
> On Wed, Apr 27, 2022 at 6:26 PM Parav Pandit <parav@nvidia.com> wrote:
> >
> > Currently when driver initiates a queue reset, device is expected
> > to communicate reset status to the driver by changing the value of the
> > queue_reset register twice. First to return value other than 1 when
> > reset is ongoing, later to return 1 when queue reset is completed.
> >
> > However initially during the device reset time the queue reset value
> > is zero. queue_reset changes the value of the register to a different
> > value on reset completion. Yet another time queue_reset value is
> > expected to change when queue_select is reprogrammed.
> >
> > For example in below flow, a created virtqueue, which is disabled
> > by driver leaves the queue state as
> > queue_enable = 0, queue_reset = 1.
> >
> > 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.


The reason I added a new item(queue_reset) at that time was because there was
such a sentence:

	The driver MUST NOT write a 0 to \field{queue_enable}.

Thanks.


>
> Thanks
>
> > c) 1,1 -> vq is enabled, driver initiated reset
> > d) 1,0 -> vq is enabled, but device is busy doing the reset
> > e) 0,1 -> vq reset is completed in the device and VQ is now disabled
> > (conflict with initial queue reset state #a above)
> >
> > On next iteration, when queue_select selects the same VQ again,
> > without enablement, device is confused to return 1 or 0 because
> > it was reset once before via queue_reset register.
> >
> > This demands complex device implementation to understand what
> > should be returned for a VQ that is reset using queue_reset register
> > vs other means.
> >
> > Instead, it is better and efficient to maintain the same VQ state
> > on the device when queue reset is completed.
> >
> > new proposed flow:
> > 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
> > (device communicates that its working on resetting VQ, consistent with #C)
> > E) 0, 0 -> q_reset by device is completed, q got disabled
> > (consistent with device init time #A)
> >
> > Hence, this patch proposes a simple change to have reset register
> > polarity to be same as that of initial reset value.
> >
> > Fixes: https://github.com/oasis-tcs/virtio-spec/issues/139
> > Fixes: 12998e738621 ("virtio: pci support virtqueue reset")
> > Fixes: a4ce81a83780 ("virtio: mmio support virtqueue reset")
> > Fixes: 3b5378d70a42 ("virtio: introduce virtqueue reset as basic facility")
> > Signed-off-by: Parav Pandit <parav@nvidia.com>
> >
> > ---
> > changelog:
> > v1->v2:
> > - Fixed below comment from Xuan Zhuo
> > - Fixed the configuration and register layout text to reflect the polarity
> > v0->v1:
> > - Fixed below comments from Michael
> > - Removed example device side pseudo code for old and new polarity
> > - Removed unwanted articles 'a' and 'the'
> > - Dropped extra empty line
> > ---
> >  content.tex | 26 ++++++++++++++++----------
> >  1 file changed, 16 insertions(+), 10 deletions(-)
> >
> > diff --git a/content.tex b/content.tex
> > index 060bdab..68b176b 100644
> > --- a/content.tex
> > +++ b/content.tex
> > @@ -1039,7 +1039,7 @@ \subsubsection{Common configuration structure layout}\label{sec:Virtio Transport
> >  \field{queue_reset} after the virtqueue is enabled with \field{queue_enable}.
> >
> >  The device MUST reset the queue when 1 is written to \field{queue_reset}, and
> > -present a 1 in \field{queue_reset} after the queue has been reset, until the
> > +present 0 in \field{queue_reset} after the queue has been reset, until the
> >  driver re-enables the queue via \field{queue_enable} or the device is reset.
> >  The device MUST present consistent default values after queue reset.
> >  (see \ref{sec:Basic Facilities of a Virtio Device / Virtqueues / Virtqueue Reset}).
> > @@ -1069,10 +1069,13 @@ \subsubsection{Common configuration structure layout}\label{sec:Virtio Transport
> >  The driver MUST NOT write a 0 to \field{queue_enable}.
> >
> >  If VIRTIO_F_RING_RESET has been negotiated, after the driver writes 1 to
> > -\field{queue_reset} to reset the queue, it MUST verify that the queue
> > -has been reset by reading back \field{queue_reset} and ensuring that it
> > -is 1. The driver MAY re-enable the queue by writing a 1 to
> > -\field{queue_enable} after ensuring that the other virtqueue fields have
> > +\field{queue_reset} to reset the queue, driver MUST verify that the queue
> > +has been reset by reading back \field{queue_reset} until device returns a value of 0.
> > +Device continue to report value of 1 for the \field{queue_reset} until device finishes
> > +the queue reset request. When the device completes the queue reset, \field{queue_reset}
> > +and \field{queue_enable} set to zero, indicating
> > +that specified queue is ready to be enabled again. The driver MAY re-enable the queue by writing 1 to
> > +\field{queue_enable} after ensuring that other virtqueue fields have
> >  been set up correctly. The driver MAY set driver-writeable queue configuration
> >  values to different values than those that were used before the queue reset.
> >  (see \ref{sec:Basic Facilities of a Virtio Device / Virtqueues / Virtqueue Reset}).
> > @@ -2015,7 +2018,7 @@ \subsection{MMIO Device Register Layout}\label{sec:Virtio Transport Options / Vi
> >  \field{QueueReset} after the virtqueue is enabled with \field{QueueReady}.
> >
> >  The device MUST reset the queue when 1 is written to \field{QueueReset}, and
> > -present a 1 in \field{QueueReset} after the queue has been reset, until the
> > +present 0 in \field{QueueReset} after the queue has been reset, until the
> >  driver re-enables the queue via \field{QueueReady} or until the device is reset.
> >  The device MUST present consistent default values after queue reset.
> >  (see \ref{sec:Basic Facilities of a Virtio Device / Virtqueues / Virtqueue Reset}).
> > @@ -2063,10 +2066,13 @@ \subsection{MMIO Device Register Layout}\label{sec:Virtio Transport Options / Vi
> >  it finishes handling an interrupt and MUST NOT set any of the undefined bits in the value.
> >
> >  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. The driver MAY re-enable the queue by writing a 1 to
> > -\field{QueueReady} after ensuring that the other virtqueue fields have
> > +\field{QueueReset} to reset the queue, driver MUST verify that the queue
> > +has been reset by reading back \field{QueueReset} until device returns a value of 0.
> > +Device continue to report value of 1 for the \field{QueueReset} until device finishes
> > +the reset. When the device completes the queue reset, \field{QueueReset} and
> > +\field{QueueReady} set to zero, indicating that specified queue is ready to be
> > +enabled again. The driver MAY re-enable the queue by writing 1 to
> > +\field{QueueReady} after ensuring that other virtqueue fields have
> >  been set up correctly. The driver MAY set driver-writeable queue configuration
> >  values to different values than those that were used before the queue reset.
> >  (see \ref{sec:Basic Facilities of a Virtio Device / Virtqueues / Virtqueue Reset}).
> > --
> > 1.8.3.1
> >
> >
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