lguest: update commentry

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Documentation/lguest/lguest.c

@@ -895,6 +895,9 @@ static void handle_console_output(int fd, struct virtqueue *vq, bool timeout)
 	}
 }
 
+/* This is called when we no longer want to hear about Guest changes to a
+ * virtqueue.  This is more efficient in high-traffic cases, but it means we
+ * have to set a timer to check if any more changes have occurred. */
 static void block_vq(struct virtqueue *vq)
 {
 	struct itimerval itm;
@@ -939,6 +942,11 @@ static void handle_net_output(int fd, struct virtqueue *vq, bool timeout)
 	if (!timeout && num)
 		block_vq(vq);
 
+	/* We never quite know how long should we wait before we check the
+	 * queue again for more packets.  We start at 500 microseconds, and if
+	 * we get fewer packets than last time, we assume we made the timeout
+	 * too small and increase it by 10 microseconds.  Otherwise, we drop it
+	 * by one microsecond every time.  It seems to work well enough. */
 	if (timeout) {
 		if (num < last_timeout_num)
 			timeout_usec += 10;

drivers/lguest/lguest_device.c

@@ -98,6 +98,10 @@ static u32 lg_get_features(struct virtio_device *vdev)
 	return features;
 }
 
+/* The virtio core takes the features the Host offers, and copies the
+ * ones supported by the driver into the vdev->features array.  Once
+ * that's all sorted out, this routine is called so we can tell the
+ * Host which features we understand and accept. */
 static void lg_finalize_features(struct virtio_device *vdev)
 {
 	unsigned int i, bits;
@@ -108,6 +112,10 @@ static void lg_finalize_features(struct virtio_device *vdev)
 	/* Give virtio_ring a chance to accept features. */
 	vring_transport_features(vdev);
 
+	/* The vdev->feature array is a Linux bitmask: this isn't the
+	 * same as a the simple array of bits used by lguest devices
+	 * for features.  So we do this slow, manual conversion which is
+	 * completely general. */
 	memset(out_features, 0, desc->feature_len);
 	bits = min_t(unsigned, desc->feature_len, sizeof(vdev->features)) * 8;
 	for (i = 0; i < bits; i++) {