3 files changed, 13 insertions, 6 deletions

diff --git a/Documentation/lguest/lguest.c b/Documentation/lguest/lguest.c
index 4df1804169dc..8ff2d8bc690a 100644
--- a/Documentation/lguest/lguest.c
+++ b/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;
 }
diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 342bb0363fbe..dbe1d35db32a 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/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;
         }
diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
index ea3be89a0e83..abe481ed990e 100644
--- a/include/linux/virtio_ring.h
+++ b/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". */