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

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 <[email protected]>

diff -r 89bf4894cc36 drivers/virtio/virtio_ring.c

drivers/virtio/virtio_ring.c +5 -2

@@ -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;
 	}

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.