[PATCH] virtio: introduce VIRTIO_F_RING_RESET for reset queue

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

This patch allows the driver to reset a queue individually.

This is very common on general network equipment. By disabling a queue,
you can quickly reclaim the buffer currently on the queue. If necessary,
we can reinitialize the queue separately.

For example, when virtio-net implements support for AF_XDP, we need to
disable a queue to release all the original buffers when AF_XDP setup.
And quickly release all the AF_XDP buffers that have been placed in the
queue when AF_XDP exits.

Since mmio's QueueReady allows setting 0 to stop the queue, there is no
need to distinguish whether VIRTIO_F_RING_RESET is negotiated.

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
---
 content.tex | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/content.tex b/content.tex
index 7cec1c3..70a9765 100644
--- a/content.tex
+++ b/content.tex
@@ -1001,7 +1001,13 @@ \subsubsection{Common configuration structure layout}\label{sec:Virtio Transport
 After writing 0 to \field{device_status}, the driver MUST wait for a read of
 \field{device_status} to return 0 before reinitializing the device.
 
-The driver MUST NOT write a 0 to \field{queue_enable}.
+If the VIRTIO_F_RING_RESET feature bit is not negotiated,
+the driver MUST NOT write a 0 to \field{queue_enable}.
+
+Otherwise, if the VIRTIO_F_RING_RESET feature bit is negotiated,
+the driver can stop using the queue by writing a 0 to \field{queue_enable} and
+MUST read the value back to ensure synchronization. Then optionally re-enable
+the queue.
 
 \subsubsection{Notification structure layout}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / Notification capability}
 
@@ -6672,6 +6678,9 @@ \chapter{Reserved Feature Bits}\label{sec:Reserved Feature Bits}
   transport specific.
   For more details about driver notifications over PCI see \ref{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI-specific Initialization And Device Operation / Available Buffer Notifications}.
 
+  \item[VIRTIO_F_RING_RESET(40)] This feature indicates
+  that the driver can reset a queue individually.
+
 \end{description}
 
 \drivernormative{\section}{Reserved Feature Bits}{Reserved Feature Bits}
-- 
2.31.0