macvtap: Perform GSO on forwarding path.

When macvtap forwards skb to its tap, it needs to check
if GSO needs to be performed.  This is sometimes necessary
when the HW device performed GRO, but the guest reading
from the tap does not support it (ex: Windows 7).
Signed-off-by: Vlad Yasevich <vyasevic@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/macvtap.c

@@ -276,14 +276,44 @@ static void macvtap_del_queues(struct net_device *dev)
  */
 static int macvtap_forward(struct net_device *dev, struct sk_buff *skb)
 {
+	struct macvlan_dev *vlan = netdev_priv(dev);
 	struct macvtap_queue *q = macvtap_get_queue(dev, skb);
+	netdev_features_t features;
 	if (!q)
 		goto drop;
 
 	if (skb_queue_len(&q->sk.sk_receive_queue) >= dev->tx_queue_len)
 		goto drop;
 
+	skb->dev = dev;
+	/* Apply the forward feature mask so that we perform segmentation
+	 * according to users wishes.
+	 */
+	features = netif_skb_features(skb) & vlan->tap_features;
+	if (netif_needs_gso(skb, features)) {
+		struct sk_buff *segs = __skb_gso_segment(skb, features, false);
+
+		if (IS_ERR(segs))
+			goto drop;
+
+		if (!segs) {
+			skb_queue_tail(&q->sk.sk_receive_queue, skb);
+			goto wake_up;
+		}
+
+		kfree_skb(skb);
+		while (segs) {
+			struct sk_buff *nskb = segs->next;
+
+			segs->next = NULL;
+			skb_queue_tail(&q->sk.sk_receive_queue, segs);
+			segs = nskb;
+		}
+	} else {
 		skb_queue_tail(&q->sk.sk_receive_queue, skb);
+	}
+
+wake_up:
 	wake_up_interruptible_poll(sk_sleep(&q->sk), POLLIN | POLLRDNORM | POLLRDBAND);
 	return NET_RX_SUCCESS;