[PATCH RFC] qemu/virtio: use last used index published by guest

["Michael S. Tsirkin" <mst@redhat.com>]

Reduces irq_window in guest by only injecting
an interrupt if guest has handled all buffers we
used so far.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---

This is the qemu part of the story.

 hw/vhost_net.c |    6 ++++++
 hw/virtio.c    |   15 +++++++++++++++
 hw/virtio.h    |    4 ++++
 3 files changed, 25 insertions(+), 0 deletions(-)

diff --git a/hw/vhost_net.c b/hw/vhost_net.c
index 2e292ee..d77c9b2 100644
--- a/hw/vhost_net.c
+++ b/hw/vhost_net.c
@@ -51,6 +51,9 @@ unsigned vhost_net_get_features(struct vhost_net *net, unsigned features)
     if (!(net->dev.features & (1 << VIRTIO_RING_F_INDIRECT_DESC))) {
         features &= ~(1 << VIRTIO_RING_F_INDIRECT_DESC);
     }
+    if (!(net->dev.features & (1 << VIRTIO_RING_F_PUBLISH_USED))) {
+        features &= ~(1 << VIRTIO_RING_F_PUBLISH_USED);
+    }
     features &= ~(1 << VIRTIO_NET_F_MRG_RXBUF);
     return features;
 }
@@ -64,6 +67,9 @@ void vhost_net_ack_features(struct vhost_net *net, unsigned features)
     if (features & (1 << VIRTIO_RING_F_INDIRECT_DESC)) {
         net->dev.acked_features |= (1 << VIRTIO_RING_F_INDIRECT_DESC);
     }
+    if (features & (1 << VIRTIO_RING_F_PUBLISH_USED)) {
+        net->dev.acked_features |= (1 << VIRTIO_RING_F_PUBLISH_USED);
+    }
 }
 
 static int vhost_net_get_fd(VLANClientState *backend)
diff --git a/hw/virtio.c b/hw/virtio.c
index e7657ae..5d686f0 100644
--- a/hw/virtio.c
+++ b/hw/virtio.c
@@ -71,6 +71,7 @@ struct VirtQueue
     target_phys_addr_t pa;
     uint16_t last_avail_idx;
     int inuse;
+    int num_notify;
     uint16_t vector;
     void (*handle_output)(VirtIODevice *vdev, VirtQueue *vq);
     VirtIODevice *vdev;
@@ -139,6 +140,11 @@ static inline uint16_t vring_avail_ring(VirtQueue *vq, int i)
     return lduw_phys(pa);
 }
 
+static inline uint16_t vring_last_used_idx(VirtQueue *vq)
+{
+    return vring_avail_ring(vq, vq->vring.num);
+}
+
 static inline void vring_used_ring_id(VirtQueue *vq, int i, uint32_t val)
 {
     target_phys_addr_t pa;
@@ -234,6 +240,7 @@ void virtqueue_flush(VirtQueue *vq, unsigned int count)
     wmb();
     vring_used_idx_increment(vq, count);
     vq->inuse -= count;
+    vq->num_notify += count;
 }
 
 void virtqueue_push(VirtQueue *vq, const VirtQueueElement *elem,
@@ -603,6 +610,14 @@ void virtio_irq(VirtQueue *vq)
 
 void virtio_notify(VirtIODevice *vdev, VirtQueue *vq)
 {
+    /* Do not notify if guest did not yet see the last update. */
+    if ((vdev->guest_features & (1 << VIRTIO_RING_F_PUBLISH_USED)) &&
+	vring_last_used_idx(vq) !=
+	(uint16_t)(vring_used_idx(vq) + vq->num_notify))
+	return;
+
+    vq->num_notify = 0;
+
     /* Always notify when queue is empty (when feature acknowledge) */
     if ((vring_avail_flags(vq) & VRING_AVAIL_F_NO_INTERRUPT) &&
         (!(vdev->guest_features & (1 << VIRTIO_F_NOTIFY_ON_EMPTY)) ||
diff --git a/hw/virtio.h b/hw/virtio.h
index f885f1b..4bdfded 100644
--- a/hw/virtio.h
+++ b/hw/virtio.h
@@ -43,6 +43,8 @@
 #define VIRTIO_F_NOTIFY_ON_EMPTY        24
 /* We support indirect buffer descriptors */
 #define VIRTIO_RING_F_INDIRECT_DESC     28
+/* The Guest publishes last-seen used index at the end of the avail ring. */
+#define VIRTIO_RING_F_PUBLISH_USED	29
 /* A guest should never accept this.  It implies negotiation is broken. */
 #define VIRTIO_F_BAD_FEATURE		30
 
@@ -189,6 +191,8 @@ VirtIODevice *virtio_balloon_init(DeviceState *dev);
 void virtio_net_exit(VirtIODevice *vdev);
 
 #define DEFINE_VIRTIO_COMMON_FEATURES(_state, _field) \
+	DEFINE_PROP_BIT("publish_used", _state, _field, \
+			VIRTIO_RING_F_PUBLISH_USED, true), \
 	DEFINE_PROP_BIT("indirect_desc", _state, _field, \
 			VIRTIO_RING_F_INDIRECT_DESC, true)
 
-- 
1.7.1.12.g42b7f