netvsc: reduce maximum GSO size

commit a50af86dd49ee1851d1ccf06dd0019c05b95e297 upstream.

Hyper-V (and Azure) support using NVGRE which requires some extra space
for encapsulation headers. Because of this the largest allowed TSO
packet is reduced.

For older releases, hard code a fixed reduced value.  For next release,
there is a better solution which uses result of host offload
negotiation.

Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Willy Tarreau <w@1wt.eu>

drivers/net/hyperv/netvsc_drv.c

@@ -48,6 +48,9 @@ struct net_device_context {
 	struct work_struct work;
 };
 
+/* Restrict GSO size to account for NVGRE */
+#define NETVSC_GSO_MAX_SIZE	62768
+
 #define RING_SIZE_MIN 64
 static int ring_size = 128;
 module_param(ring_size, int, S_IRUGO);
@@ -436,6 +439,7 @@ static int netvsc_probe(struct hv_device *dev,
 
 	SET_ETHTOOL_OPS(net, &ethtool_ops);
 	SET_NETDEV_DEV(net, &dev->device);
+	netif_set_gso_max_size(net, NETVSC_GSO_MAX_SIZE);
 
 	ret = register_netdev(net);
 	if (ret != 0) {