Add support for postponing VirtIO virtqueue interrupts

Partial support for the EVENT_IDX feature was added a while ago,
but this commit adds an interface for the device driver to hint
how long (in terms of descriptors) the next interrupt should be
delayed.

The first user of this will be used to reduce VirtIO net's Tx
completion interrupts.

2 files changed, 32 insertions, 12 deletions

diff --git a/sys/dev/virtio/virtqueue.c b/sys/dev/virtio/virtqueue.c
index a82426e..beff14c 100644
--- a/sys/dev/virtio/virtqueue.c
+++ b/sys/dev/virtio/virtqueue.c
@@ -127,7 +127,7 @@ static uint16_t	vq_ring_enqueue_segments(struct virtqueue *,
 static int	vq_ring_use_indirect(struct virtqueue *, int);
 static void	vq_ring_enqueue_indirect(struct virtqueue *, void *,
 		    struct sglist *, int, int);
-static int 	vq_ring_enable_interrupt(struct virtqueue *, uint16_t);
+static int	vq_ring_enable_interrupt(struct virtqueue *, uint16_t);
 static int	vq_ring_must_notify_host(struct virtqueue *);
 static void	vq_ring_notify_host(struct virtqueue *);
 static void	vq_ring_free_chain(struct virtqueue *, uint16_t);
@@ -440,28 +440,38 @@ virtqueue_enable_intr(struct virtqueue *vq)
 }
 
 int
-virtqueue_postpone_intr(struct virtqueue *vq)
+virtqueue_postpone_intr(struct virtqueue *vq, vq_postpone_t hint)
 {
 	uint16_t ndesc, avail_idx;
 
-	/*
-	 * Request the next interrupt be postponed until at least half
-	 * of the available descriptors have been consumed.
-	 */
 	avail_idx = vq->vq_ring.avail->idx;
-	ndesc = (uint16_t)(avail_idx - vq->vq_used_cons_idx) / 2;
+	ndesc = (uint16_t)(avail_idx - vq->vq_used_cons_idx);
+
+	switch (hint) {
+	case VQ_POSTPONE_SHORT:
+		ndesc /= 4;
+		break;
+	case VQ_POSTPONE_LONG:
+		ndesc *= 3 / 4;
+		break;
+	case VQ_POSTPONE_EMPTIED:
+		break;
+	}
 
 	return (vq_ring_enable_interrupt(vq, ndesc));
 }
 
+/*
+ * Note this is only considered a hint to the host.
+ */
 void
 virtqueue_disable_intr(struct virtqueue *vq)
 {
 
-	/*
-	 * Note this is only considered a hint to the host.
-	 */
-	if ((vq->vq_flags & VIRTQUEUE_FLAG_EVENT_IDX) == 0)
+	if (vq->vq_flags & VIRTQUEUE_FLAG_EVENT_IDX) {
+		vring_used_event(&vq->vq_ring) = vq->vq_used_cons_idx -
+		    vq->vq_nentries - 1;
+	} else
 		vq->vq_ring.avail->flags |= VRING_AVAIL_F_NO_INTERRUPT;
 }
 
diff --git a/sys/dev/virtio/virtqueue.h b/sys/dev/virtio/virtqueue.h
index 128a10a..0d4ed94 100644
--- a/sys/dev/virtio/virtqueue.h
+++ b/sys/dev/virtio/virtqueue.h
@@ -41,6 +41,16 @@ struct sglist;
 /* Device callback for a virtqueue interrupt. */
 typedef void virtqueue_intr_t(void *);
 
+/*
+ * Hint on how long the next interrupt should be postponed. This is
+ * only used when the EVENT_IDX feature is negotiated.
+ */
+typedef enum {
+	VQ_POSTPONE_SHORT,
+	VQ_POSTPONE_LONG,
+	VQ_POSTPONE_EMPTIED	/* Until all available desc are used. */
+} vq_postpone_t;
+
 #define VIRTQUEUE_MAX_NAME_SZ	32
 
 /* One for each virtqueue the device wishes to allocate. */
@@ -73,7 +83,7 @@ int	 virtqueue_reinit(struct virtqueue *vq, uint16_t size);
 int	 virtqueue_intr_filter(struct virtqueue *vq);
 void	 virtqueue_intr(struct virtqueue *vq);
 int	 virtqueue_enable_intr(struct virtqueue *vq);
-int	 virtqueue_postpone_intr(struct virtqueue *vq);
+int	 virtqueue_postpone_intr(struct virtqueue *vq, vq_postpone_t hint);
 void	 virtqueue_disable_intr(struct virtqueue *vq);
 
 /* Get physical address of the virtqueue ring. */