[PATCH v1] virtio: Improve queue_reset polarity to match to default reset state

[Parav Pandit <parav@nvidia.com>]

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) 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:
v0->v1:
- 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..fc314ed 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 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.
 (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 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.
 (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