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>

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>
Rusty Russell
1 parent 3309daaad7
Showing 3 changed files with 13 additions and 6 deletions Side-by-side Diff
Documentation/lguest/lguest.c
drivers/virtio/virtio_ring.c
include/linux/virtio_ring.h
@@ -923,10 +923,10 @@
 	/* 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 @@
 	 * 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;
 }
@@ -87,6 +87,8 @@
 	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;
 	}
@@ -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". */
...	...	@@ -923,10 +923,10 @@
923	923	/* Check each virtqueue. */
924	924	for (i = devices.dev; i; i = i->next) {
925	925	for (vq = i->vq; vq; vq = vq->next) {
926		- if (vq->config.pfn == addr/getpagesize()
927		- && vq->handle_output) {
	926	+ if (vq->config.pfn == addr/getpagesize()) {
928	927	verbose("Output to %s\n", vq->dev->name);
929		- vq->handle_output(fd, vq);
	928	+ if (vq->handle_output)
	929	+ vq->handle_output(fd, vq);
930	930	return;
931	931	}
932	932	}
...	...	@@ -1068,7 +1068,8 @@
1068	1068	* virtqueue. */
1069	1069	vq->handle_output = handle_output;
1070	1070
1071		- /* Set the "Don't Notify Me" flag if we don't have a handler */
	1071	+ /* As an optimization, set the advisory "Don't Notify Me" flag if we
	1072	+ * don't have a handler */
1072	1073	if (!handle_output)
1073	1074	vq->vring.used->flags = VRING_USED_F_NO_NOTIFY;
1074	1075	}
...	...	@@ -87,6 +87,8 @@
87	87	if (vq->num_free < out + in) {
88	88	pr_debug("Can't add buf len %i - avail = %i\n",
89	89	out + in, vq->num_free);
	90	+ /* We notify even if VRING_USED_F_NO_NOTIFY is set here. */
	91	+ vq->notify(&vq->vq);
90	92	END_USE(vq);
91	93	return -ENOSPC;
92	94	}
...	...	@@ -15,9 +15,13 @@
15	15	/* This marks a buffer as write-only (otherwise read-only). */
16	16	#define VRING_DESC_F_WRITE 2
17	17
18		-/* This means don't notify other side when buffer added. */
	18	+/* The Host uses this in used->flags to advise the Guest: don't kick me when
	19	+ * you add a buffer. It's unreliable, so it's simply an optimization. Guest
	20	+ * will still kick if it's out of buffers. */
19	21	#define VRING_USED_F_NO_NOTIFY 1
20		-/* This means don't interrupt guest when buffer consumed. */
	22	+/* The Guest uses this in avail->flags to advise the Host: don't interrupt me
	23	+ * when you consume a buffer. It's unreliable, so it's simply an
	24	+ * optimization. */
21	25	#define VRING_AVAIL_F_NO_INTERRUPT 1
22	26
23	27	/* Virtio ring descriptors: 16 bytes. These can chain together via "next". */