virtio_net: disable cb aggressively

There are currently two cases where we poll TX vq not in response to a
callback: start xmit and rx napi.  We currently do this with callbacks
enabled which can cause extra interrupts from the card.  Used not to be
a big issue as we run with interrupts disabled but that is no longer the
case, and in some cases the rate of spurious interrupts is so high
linux detects this and actually kills the interrupt.

Fix up by disabling the callbacks before polling the tx vq.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/net/virtio_net.c

@@ -1516,7 +1516,10 @@ static void virtnet_poll_cleantx(struct receive_queue *rq)
 		return;
 
 	if (__netif_tx_trylock(txq)) {
+		do {
+			virtqueue_disable_cb(sq->vq);
 			free_old_xmit_skbs(sq, true);
+		} while (unlikely(!virtqueue_enable_cb_delayed(sq->vq)));
 
 		if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS)
 			netif_tx_wake_queue(txq);
@@ -1691,10 +1694,14 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
 	bool use_napi = sq->napi.weight;
 
 	/* Free up any pending old buffers before queueing new ones. */
+	do {
+		if (use_napi)
+			virtqueue_disable_cb(sq->vq);
+
 		free_old_xmit_skbs(sq, false);
 
-	if (use_napi && kick)
-		virtqueue_enable_cb_delayed(sq->vq);
+	} while (use_napi && kick &&
+	       unlikely(!virtqueue_enable_cb_delayed(sq->vq)));
 
 	/* timestamp packet in software */
 	skb_tx_timestamp(skb);