datapath: stt: Relax MTU constraints.

Currently, even if the entire path supports jumbo frames, the STT netdev
limits the path MTU to 1500 bytes, and cannot be configured otherwise.
Relax the constraints on modifying the device MTU, and set it to the
maximum by default.

Signed-off-by: Joe Stringer <joe@ovn.org>
Acked-by: Jesse Gross <jesse@kernel.org>

datapath/linux/compat/stt.c

@@ -1683,14 +1683,38 @@ static int stt_stop(struct net_device *dev)
 	return 0;
 }
 
+static int __stt_change_mtu(struct net_device *dev, int new_mtu, bool strict)
+{
+	int max_mtu = IP_MAX_MTU - STT_HEADER_LEN - sizeof(struct iphdr)
+		      - dev->hard_header_len;
+
+	if (new_mtu < 68)
+		return -EINVAL;
+
+	if (new_mtu > max_mtu) {
+		if (strict)
+			return -EINVAL;
+
+		new_mtu = max_mtu;
+	}
+
+	dev->mtu = new_mtu;
+	return 0;
+}
+
+static int stt_change_mtu(struct net_device *dev, int new_mtu)
+{
+	return __stt_change_mtu(dev, new_mtu, true);
+}
+
 static const struct net_device_ops stt_netdev_ops = {
 	.ndo_init               = stt_init,
 	.ndo_uninit             = stt_uninit,
 	.ndo_open               = stt_open,
 	.ndo_stop               = stt_stop,
 	.ndo_start_xmit         = stt_dev_xmit,
 	.ndo_get_stats64        = ip_tunnel_get_stats64,
-	.ndo_change_mtu         = eth_change_mtu,
+	.ndo_change_mtu         = stt_change_mtu,
 	.ndo_validate_addr      = eth_validate_addr,
 	.ndo_set_mac_address    = eth_mac_addr,
 };
@@ -1782,6 +1806,10 @@ static int stt_configure(struct net *net, struct net_device *dev,
 	if (find_dev(net, dst_port))
 		return -EBUSY;
 
+	err = __stt_change_mtu(dev, IP_MAX_MTU, false);
+	if (err)
+		return err;
+
 	err = register_netdevice(dev);
 	if (err)
 		return err;