xen-netback: process guest rx packets in batches

Instead of only placing one skb on the guest rx ring at a time, process
a batch of up-to 64.  This improves performance by ~10% in some tests.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
[re-based]
Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/xen-netback/rx.c

@@ -399,7 +399,7 @@ static void xenvif_rx_extra_slot(struct xenvif_queue *queue,
 	BUG();
 }
 
-void xenvif_rx_action(struct xenvif_queue *queue)
+void xenvif_rx_skb(struct xenvif_queue *queue)
 {
 	struct xenvif_pkt_state pkt;
 
@@ -425,6 +425,19 @@ void xenvif_rx_action(struct xenvif_queue *queue)
 	xenvif_rx_complete(queue, &pkt);
 }
 
+#define RX_BATCH_SIZE 64
+
+void xenvif_rx_action(struct xenvif_queue *queue)
+{
+	unsigned int work_done = 0;
+
+	while (xenvif_rx_ring_slots_available(queue) &&
+	       work_done < RX_BATCH_SIZE) {
+		xenvif_rx_skb(queue);
+		work_done++;
+	}
+}
+
 static bool xenvif_rx_queue_stalled(struct xenvif_queue *queue)
 {
 	RING_IDX prod, cons;