Re: [PATCH v4] virtio: Improve queue_reset polarity to match to default reset state

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

On Fri, Apr 29, 2022 at 10:07:04AM +0200, Cornelia Huck wrote:
> On Fri, Apr 29 2022, 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
> > c) 1,1 -> vq is enabled, driver initiated reset
> > d) 0,0 -> queue reset is ongoing
> >    conflicts with #a, because queue is still enabled in device
> >    whose reset is in progress.
> >    External entity has no knowledge if VQ is undergoing reset or
> >    VQ is never enabled.
> > e) 0,1 -> queue reset is completed (conflicts with #a initial value)
> >
> > 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.
> >
> > Additionally, external entity that may have to manage a virtio device
> > cannot distinguish between #a and #d, whether a VQ is not yet enabled
> > or it is in use and undergoing a reset.
> >
> > 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:
> > v3->v4:
> > - removed incorrect requirement to read queue_enable on resetting
> >   the queue
> > v2->v3:
> > - Updated the commit log to capture the problem more accurately
> > - Fixed below comments from Michael and Cornelia
> > - Fixed adding article 'the' before driver
> > - Change wording in driver requirements section to reflect driver
> >   requirements reflecting device behavior
> > - Added device compliance statement on what to return for queue_reset
> >   register to indicate ongoing reset in progress
> > - Moved device specific description adjacent to other device related
> >   queue reset description
> > 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 | 48 ++++++++++++++++++++----------------------------
> >  1 file changed, 20 insertions(+), 28 deletions(-)
> >
> > diff --git a/content.tex b/content.tex
> > index 060bdab..fdd255d 100644
> > --- a/content.tex
> > +++ b/content.tex
> > @@ -1029,19 +1029,15 @@ \subsubsection{Common configuration structure layout}\label{sec:Virtio Transport
> >  The device MUST present a 0 in \field{queue_enable} on reset.
> >  
> >  If VIRTIO_F_RING_RESET has been negotiated, the device MUST present a 0 in
> > -\field{queue_enable} after the driver has reset the virtqueue via
> > -\field{queue_reset}.
> > -
> > -If VIRTIO_F_RING_RESET has been negotiated, the device MUST present a 0 in
> >  \field{queue_reset} on reset.
> >  
> >  If VIRTIO_F_RING_RESET has been negotiated, the device MUST present a 0 in
> >  \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
> > -driver re-enables the queue via \field{queue_enable} or the device is reset.
> > -The device MUST present consistent default values after queue reset.
> > +The device MUST reset the queue when 1 is written to \field{queue_reset}. The
> > +device MUST continue to present 1 in \field{queue_reset} when the queue reset
> 
> s/when/as long as/


yes that is better than my proposed wording


> > +is ongoing. The device MUST present 0 in \field{queue_reset} and \field{queue_enable}
> 
> s/in/in both/
> 
> > +when queue reset completes.
> 
> Maybe "when queue reset has completed"?
> 
> >  (see \ref{sec:Basic Facilities of a Virtio Device / Virtqueues / Virtqueue Reset}).
> >  
> >  The device MUST present a 0 in \field{queue_size} if the virtqueue
> > @@ -1069,12 +1065,12 @@ \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
> > -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.
> > +\field{queue_reset} to reset the queue, the driver MUST NOT consider queue
> > +reset to be complete until it reads back 0 in \field{queue_reset}. 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}).
> >  
> >  \subsubsection{Notification structure layout}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / Notification capability}
> > @@ -2005,19 +2001,15 @@ \subsection{MMIO Device Register Layout}\label{sec:Virtio Transport Options / Vi
> >  The device MUST NOT access virtual queue contents when \field{QueueReady} is zero (0x0).
> >  
> >  If VIRTIO_F_RING_RESET has been negotiated, the device MUST present a 0 in
> > -\field{QueueReady} after the driver has reset the virtqueue via
> > -\field{QueueReset}.
> > -
> > -If VIRTIO_F_RING_RESET has been negotiated, the device MUST present a 0 in
> >  \field{QueueReset} on reset.
> >  
> >  If VIRTIO_F_RING_RESET has been negotiated, The device MUST present a 0 in
> >  \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
> > -driver re-enables the queue via \field{QueueReady} or until the device is reset.
> > -The device MUST present consistent default values after queue reset.
> > +The device MUST reset the queue when 1 is written to \field{QueueReset}. The
> > +device MUST continue to present 1 in \field{QueueReset} when the queue reset
> 
> s/when/as long as/
> 
> > +is ongoing. The device MUST present 0 in \field{QueueReset} and \field{QueueReady}
> 
> s/in/in both/
> 
> > +when queue reset completes.
> 
> Maybe "has completed" here as well.
> 
> >  (see \ref{sec:Basic Facilities of a Virtio Device / Virtqueues / Virtqueue Reset}).
> >  
> >  \drivernormative{\subsubsection}{MMIO Device Register Layout}{Virtio Transport Options / Virtio Over MMIO / MMIO Device Register Layout}
> > @@ -2063,12 +2055,12 @@ \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
> > -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.
> > +\field{QueueReset} to reset the queue, the driver MUST NOT consider queue
> > +reset to be complete until it reads back 0 in \field{QueueReset}. 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}).
> >  
> >  \subsection{MMIO-specific Initialization And Device Operation}\label{sec:Virtio Transport Options / Virtio Over MMIO / MMIO-specific Initialization And Device Operation}


ack on these proposed changes