Re: [virtio-comment] [PATCH v3] virtio: Improve queue_reset polarity to match to default reset state

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

On Thu, Apr 28, 2022 at 04:50:41AM +0300, Parav Pandit 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:
> 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 | 44 ++++++++++++++++++--------------------------
>  1 file changed, 18 insertions(+), 26 deletions(-)
> 
> diff --git a/content.tex b/content.tex
> index 060bdab..c0b3f15 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
> +is ongoing. The device MUST present 0 in \field{queue_reset} and \field{queue_enable}
> +when queue reset completes.
>  (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
> @@ -1069,11 +1065,11 @@ \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
> +\field{queue_reset} to reset the queue, the driver MUST NOT consider queue
> +reset to be complete until it reads back 0 in both \field{queue_reset} and
> +\field{queue_enable}.

two reads are now required, this is unfortunate.
why is it not enough to read back 0 in 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}).
>  
> @@ -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
> +is ongoing. The device MUST present 0 in \field{QueueReset} and \field{QueueReady}
> +when queue reset completes.
>  (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}
> @@ -2063,11 +2055,11 @@ \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
> +\field{QueueReset} to reset the queue, the driver MUST NOT consider queue
> +reset to be complete until it reads back 0 in both \field{QueueReset} and
> +\field{QueueReady}. 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
> 
> 
> This publicly archived list offers a means to provide input to the
> OASIS Virtual I/O Device (VIRTIO) TC.
> 
> In order to verify user consent to the Feedback License terms and
> to minimize spam in the list archive, subscription is required
> before posting.
> 
> Subscribe: virtio-comment-subscribe@lists.oasis-open.org
> Unsubscribe: virtio-comment-unsubscribe@lists.oasis-open.org
> List help: virtio-comment-help@lists.oasis-open.org
> List archive: https://lists.oasis-open.org/archives/virtio-comment/
> Feedback License: https://www.oasis-open.org/who/ipr/feedback_license.pdf
> List Guidelines: https://www.oasis-open.org/policies-guidelines/mailing-lists
> Committee: https://www.oasis-open.org/committees/virtio/
> Join OASIS: https://www.oasis-open.org/join/