virtio: clarify NO_NOTIFY flag usage

The other side (host) can set the NO_NOTIFY flag as an optimization,
to say "no need to kick me when you add things".  Make it clear that
this is advisory only; especially that we should always notify when
the ring is full.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Documentation/lguest/lguest.c

@@ -923,10 +923,10 @@ static void handle_output(int fd, unsigned long addr)
 	/* Check each virtqueue. */
 	for (i = devices.dev; i; i = i->next) {
 		for (vq = i->vq; vq; vq = vq->next) {
-			if (vq->config.pfn == addr/getpagesize()
-			    && vq->handle_output) {
+			if (vq->config.pfn == addr/getpagesize()) {
 				verbose("Output to %s\n", vq->dev->name);
-				vq->handle_output(fd, vq);
+				if (vq->handle_output)
+					vq->handle_output(fd, vq);
 				return;
 			}
 		}
@@ -1068,7 +1068,8 @@ static void add_virtqueue(struct device *dev, unsigned int num_descs,
 	 * virtqueue. */
 	vq->handle_output = handle_output;
 
-	/* Set the "Don't Notify Me" flag if we don't have a handler */
+	/* As an optimization, set the advisory "Don't Notify Me" flag if we
+	 * don't have a handler */
 	if (!handle_output)
 		vq->vring.used->flags = VRING_USED_F_NO_NOTIFY;
 }

drivers/virtio/virtio_ring.c

@@ -87,6 +87,8 @@ static int vring_add_buf(struct virtqueue *_vq,
 	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);
 		END_USE(vq);
 		return -ENOSPC;
 	}

include/linux/virtio_ring.h

@@ -15,9 +15,13 @@
 /* This marks a buffer as write-only (otherwise read-only). */
 #define VRING_DESC_F_WRITE	2
 
-/* This means don't notify other side when buffer added. */
+/* The Host uses this in used->flags to advise the Guest: don't kick me when
+ * you add a buffer.  It's unreliable, so it's simply an optimization.  Guest
+ * will still kick if it's out of buffers. */
 #define VRING_USED_F_NO_NOTIFY	1
-/* This means don't interrupt guest when buffer consumed. */
+/* The Guest uses this in avail->flags to advise the Host: don't interrupt me
+ * when you consume a buffer.  It's unreliable, so it's simply an
+ * optimization.  */
 #define VRING_AVAIL_F_NO_INTERRUPT	1
 
 /* Virtio ring descriptors: 16 bytes.  These can chain together via "next". */