[PATCH] virtio: don't always force a notification when ring is full

[Rusty Russell <rusty@rustcorp.com.au>]

We force notification when the ring is full, even if the host has
indicated it doesn't want to know.  This seemed like a good idea at
the time: if we fill the transmit ring, we should tell the host
immediately.

Unfortunately this logic also applies to the receiving ring, which is
refilled constantly.  We should introduce real notification thesholds
to replace this logic.  Meanwhile, removing the logic altogether breaks
the heuristics which KVM uses, so we use a hack: only notify if there are
outgoing parts of the new buffer.

It even makes a difference with lguest's crappy network implementation:
	1GB guest->host before: 30.0844
	1GB guest->host after: 27.6167

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

diff -r 89bf4894cc36 drivers/virtio/virtio_ring.c
--- a/drivers/virtio/virtio_ring.c	Mon Jun 16 14:31:30 2008 +1000
+++ b/drivers/virtio/virtio_ring.c	Wed Jun 18 16:15:49 2008 +1000
@@ -87,8 +87,11 @@ static int vring_add_buf(struct virtqueu
 	if (vq->num_free < out + in) {
 		pr_debug("Can't add buf len %i - avail = %i\n",
 			 out + in, vq->num_free);
-		/* We notify *even if* VRING_USED_F_NO_NOTIFY is set here. */
-		vq->notify(&vq->vq);
+		/* FIXME: for historical reasons, we force a notify here if
+		 * there are outgoing parts to the buffer.  Presumably the
+		 * host should service the ring ASAP. */
+		if (out)
+			vq->notify(&vq->vq);
 		END_USE(vq);
 		return -ENOSPC;
 	}