virtio-net: correct capacity math on ring full

Capacity math on ring full is wrong: we are
looking at num_sg but that might be optimistic
because of indirect buffer use.

The implementation also penalizes fast path
with extra memory accesses for the benefit of
ring full condition handling which is slow path.

It's easy to query ring capacity so let's do just that.

This change also makes it easier to move vnet header
for tx around as follow-up patch does.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Acked-by: Michael S. Tsirkin <mst@redhat.com>

drivers/net/virtio_net.c

@@ -557,10 +557,10 @@ again:
 	return received;
 }
 
-static unsigned int free_old_xmit_skbs(struct virtnet_info *vi)
+static void free_old_xmit_skbs(struct virtnet_info *vi)
 {
 	struct sk_buff *skb;
-	unsigned int len, tot_sgs = 0;
+	unsigned int len;
 	struct virtnet_stats *stats = this_cpu_ptr(vi->stats);
 
 	while ((skb = virtqueue_get_buf(vi->svq, &len)) != NULL) {
@@ -571,10 +571,8 @@ static unsigned int free_old_xmit_skbs(struct virtnet_info *vi)
 		stats->tx_packets++;
 		u64_stats_update_end(&stats->tx_syncp);
 
-		tot_sgs += skb_vnet_hdr(skb)->num_sg;
 		dev_kfree_skb_any(skb);
 	}
-	return tot_sgs;
 }
 
 static int xmit_skb(struct virtnet_info *vi, struct sk_buff *skb)
@@ -664,7 +662,8 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
 		netif_stop_queue(dev);
 		if (unlikely(!virtqueue_enable_cb_delayed(vi->svq))) {
 			/* More just got used, free them then recheck. */
-			capacity += free_old_xmit_skbs(vi);
+			free_old_xmit_skbs(vi);
+			capacity = vi->svq->num_free;
 			if (capacity >= 2+MAX_SKB_FRAGS) {
 				netif_start_queue(dev);
 				virtqueue_disable_cb(vi->svq);