virtio: Allow guest to defer VIRTIO_F_NOTIFY_ON_EMPTY (Alex Williamson)

There may be cases where the guest does not want the avail queue
interrupt, even when it's empty.  For the virtio-net case, the
guest may use a different buffering scheme or decide polling for
used buffers is more efficient.  This can be accomplished by simply
checking for whether the guest has acknowledged the existing notify
on empty flag.

Signed-off-by: Alex Williamson <alex.williamson@hp.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
git-svn-id: svn://svn.savannah.nongnu.org/qemu/branches/stable_0_10_0@6868 c046a42c-6fe2-441c-8c8c-71466251a162

diff --git a/hw/virtio.c b/hw/virtio.c
index b94ab0f0547cb5d8e5649e68aa45e4f7166f4f61..08ea16dee5024e19e0815252676bc28d8c8d2f4a 100644 (file)
--- a/hw/virtio.c
+++ b/hw/virtio.c
@@ -726,9 +726,10 @@ VirtQueue *virtio_add_queue(VirtIODevice *vdev, int queue_size,
 
 void virtio_notify(VirtIODevice *vdev, VirtQueue *vq)
 {
-    /* Always notify when queue is empty */
-    if ((vq->inuse || vring_avail_idx(vq) != vq->last_avail_idx) &&
-        (vring_avail_flags(vq) & VRING_AVAIL_F_NO_INTERRUPT))
+    /* Always notify when queue is empty (when feature acknowledge) */
+    if ((vring_avail_flags(vq) & VRING_AVAIL_F_NO_INTERRUPT) &&
+        (!(vdev->features & (1 << VIRTIO_F_NOTIFY_ON_EMPTY)) ||
+         (vq->inuse || vring_avail_idx(vq) != vq->last_avail_idx)))
         return;
 
     vdev->isr |= 0x01;