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>

1 files changed, 9 insertions, 5 deletions

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index a829930dac15..63c7810e1545 100644
--- a/drivers/net/virtio_net.c
+++ b/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);