[PATCH RFC v5 net-next 1/6] virtio_ring: fix virtqueue_enable_cb() when only 1 buffers were pending

From: Jason Wang <jasowang@redhat.com>
To: netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
	virtualization@lists.linux-foundation.org, mst@redhat.com
Cc: pagupta@redhat.com, Jason Wang <jasowang@redhat.com>
Subject: [PATCH RFC v5 net-next 1/6] virtio_ring: fix virtqueue_enable_cb() when only 1 buffers were pending
Date: Mon,  9 Feb 2015 03:39:20 -0500	[thread overview]
Message-ID: <1423471165-34243-2-git-send-email-jasowang@redhat.com> (raw)
In-Reply-To: <1423471165-34243-1-git-send-email-jasowang@redhat.com>

We currently does:

bufs = (avail->idx - last_used_idx) * 3 / 4;

This is ok now since we only try to enable the delayed callbacks when
the queue is about to be full. This may not work well when there is
only one pending buffer in the virtqueue (this may be the case after
tx interrupt was enabled). Since virtqueue_enable_cb() will return
false which may cause unnecessary triggering of napis. This patch
correct this by only calculate the four thirds when bufs is not one.

Signed-off-by: Jason Wang <jasowang@redhat.com>
---
 drivers/virtio/virtio_ring.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 00ec6b3..545fed5 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -636,7 +636,10 @@ bool virtqueue_enable_cb_delayed(struct virtqueue *_vq)
 	 * entry. Always do both to keep code simple. */
 	vq->vring.avail->flags &= cpu_to_virtio16(_vq->vdev, ~VRING_AVAIL_F_NO_INTERRUPT);
 	/* TODO: tune this threshold */
-	bufs = (u16)(virtio16_to_cpu(_vq->vdev, vq->vring.avail->idx) - vq->last_used_idx) * 3 / 4;
+	bufs = (u16)(virtio16_to_cpu(_vq->vdev, vq->vring.avail->idx) -
+		                     vq->last_used_idx);
+	if (bufs != 1)
+		bufs = bufs * 3 / 4;
 	vring_used_event(&vq->vring) = cpu_to_virtio16(_vq->vdev, vq->last_used_idx + bufs);
 	virtio_mb(vq->weak_barriers);
 	if (unlikely((u16)(virtio16_to_cpu(_vq->vdev, vq->vring.used->idx) - vq->last_used_idx) > bufs)) {
-- 
1.8.3.1

