Patch "virtio_ring: Make interrupt suppression spec compliant" has been added to the 4.8-stable tree

[<gregkh@linuxfoundation.org>]

This is a note to let you know that I've just added the patch titled

    virtio_ring: Make interrupt suppression spec compliant

to the 4.8-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     virtio_ring-make-interrupt-suppression-spec-compliant.patch
and it can be found in the queue-4.8 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From 0ea1e4a6d9b62cf29e210d2b4ba9fd43917522e3 Mon Sep 17 00:00:00 2001
From: Ladi Prosek <lprosek@redhat.com>
Date: Wed, 31 Aug 2016 14:00:04 +0200
Subject: virtio_ring: Make interrupt suppression spec compliant

From: Ladi Prosek <lprosek@redhat.com>

commit 0ea1e4a6d9b62cf29e210d2b4ba9fd43917522e3 upstream.

According to the spec, if the VIRTIO_RING_F_EVENT_IDX feature bit is
negotiated the driver MUST set flags to 0. Not dirtying the available
ring in virtqueue_disable_cb also has a minor positive performance
impact, improving L1 dcache load missed by ~0.5% in vring_bench.

Writes to the used event field (vring_used_event) are still unconditional.

Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Ladi Prosek <lprosek@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 drivers/virtio/virtio_ring.c |   14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -732,7 +732,8 @@ void virtqueue_disable_cb(struct virtque
 
 	if (!(vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT)) {
 		vq->avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
-		vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev, vq->avail_flags_shadow);
+		if (!vq->event)
+			vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev, vq->avail_flags_shadow);
 	}
 
 }
@@ -764,7 +765,8 @@ unsigned virtqueue_enable_cb_prepare(str
 	 * entry. Always do both to keep code simple. */
 	if (vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT) {
 		vq->avail_flags_shadow &= ~VRING_AVAIL_F_NO_INTERRUPT;
-		vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev, vq->avail_flags_shadow);
+		if (!vq->event)
+			vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev, vq->avail_flags_shadow);
 	}
 	vring_used_event(&vq->vring) = cpu_to_virtio16(_vq->vdev, last_used_idx = vq->last_used_idx);
 	END_USE(vq);
@@ -832,10 +834,11 @@ bool virtqueue_enable_cb_delayed(struct
 	 * more to do. */
 	/* Depending on the VIRTIO_RING_F_USED_EVENT_IDX feature, we need to
 	 * either clear the flags bit or point the event index at the next
-	 * entry. Always do both to keep code simple. */
+	 * entry. Always update the event index to keep code simple. */
 	if (vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT) {
 		vq->avail_flags_shadow &= ~VRING_AVAIL_F_NO_INTERRUPT;
-		vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev, vq->avail_flags_shadow);
+		if (!vq->event)
+			vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev, vq->avail_flags_shadow);
 	}
 	/* TODO: tune this threshold */
 	bufs = (u16)(vq->avail_idx_shadow - vq->last_used_idx) * 3 / 4;
@@ -953,7 +956,8 @@ struct virtqueue *__vring_new_virtqueue(
 	/* No callback?  Tell other side not to bother us. */
 	if (!callback) {
 		vq->avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
-		vq->vring.avail->flags = cpu_to_virtio16(vdev, vq->avail_flags_shadow);
+		if (!vq->event)
+			vq->vring.avail->flags = cpu_to_virtio16(vdev, vq->avail_flags_shadow);
 	}
 
 	/* Put everything in free lists. */


Patches currently in stable-queue which might be from lprosek@redhat.com are

queue-4.8/virtio_ring-make-interrupt-suppression-spec-compliant.patch