virtio: document queue state logic

commit d631b94e7a15277858ec5f88d674d93080506999
    virtio: change comment in transmit

started clarifying the logic behind queue state management,
but introduced an inaccuracy: TX_BUSY does not cause
a BUG message.

Clean this up some more, explaining the tradeoffs in detail.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/virtio_net.c

@@ -939,11 +939,15 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
 	skb_orphan(skb);
 	nf_reset(skb);
 
-	/* It is better to stop queue if running out of space
-	 * instead of forcing queuing layer to requeue the skb
-	 * by returning TX_BUSY (and cause a BUG message).
-	 * Since most packets only take 1 or 2 ring slots
-	 * this means 16 slots are typically wasted.
+	/* If running out of space, stop queue to avoid getting packets that we
+	 * are then unable to transmit.
+	 * An alternative would be to force queuing layer to requeue the skb by
+	 * returning NETDEV_TX_BUSY. However, NETDEV_TX_BUSY should not be
+	 * returned in a normal path of operation: it means that driver is not
+	 * maintaining the TX queue stop/start state properly, and causes
+	 * the stack to do a non-trivial amount of useless work.
+	 * Since most packets only take 1 or 2 ring slots, stopping the queue
+	 * early means 16 slots are typically wasted.
 	 */
 	if (sq->vq->num_free < 2+MAX_SKB_FRAGS) {
 		netif_stop_subqueue(dev, qnum);