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>

1 files changed, 5 insertions, 4 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;
 }