xen-netfront: request Tx response events more often

Trying to batch Tx response events results in poor performance because
this delays freeing the transmitted skbs.

Instead use the standard RING_FINAL_CHECK_FOR_RESPONSES() macro to be
notified once the next Tx response is placed on the ring.
Signed-off-by: NMalcolm Crossley <malcolm.crossley@citrix.com>
Signed-off-by: NDavid Vrabel <david.vrabel@citrix.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/xen-netfront.c

@@ -364,6 +364,7 @@ static void xennet_tx_buf_gc(struct netfront_queue *queue)
 	RING_IDX cons, prod;
 	unsigned short id;
 	struct sk_buff *skb;
+	bool more_to_do;
 
 	BUG_ON(!netif_carrier_ok(queue->info->netdev));
 
@@ -398,18 +399,8 @@ static void xennet_tx_buf_gc(struct netfront_queue *queue)
 
 		queue->tx.rsp_cons = prod;
 
-		/*
-		 * Set a new event, then check for race with update of tx_cons.
-		 * Note that it is essential to schedule a callback, no matter
-		 * how few buffers are pending. Even if there is space in the
-		 * transmit ring, higher layers may be blocked because too much
-		 * data is outstanding: in such cases notification from Xen is
-		 * likely to be the only kick that we'll get.
-		 */
-		queue->tx.sring->rsp_event =
-			prod + ((queue->tx.sring->req_prod - prod) >> 1) + 1;
-		mb();		/* update shared area */
-	} while ((cons == prod) && (prod != queue->tx.sring->rsp_prod));
+		RING_FINAL_CHECK_FOR_RESPONSES(&queue->tx, more_to_do);
+	} while (more_to_do);
 
 	xennet_maybe_wake_tx(queue);
 }