udp: intoduce udp_encap_needed static_key

Most machines dont use UDP encapsulation (L2TP)

Adds a static_key so that udp_queue_rcv_skb() doesnt have to perform a
test if L2TP never setup the encap_rcv on a socket.

Idea of this patch came after Simon Horman proposal to add a hook on TCP
as well.

If static_key is not yet enabled, the fast path does a single JMP .

When static_key is enabled, JMP destination is patched to reach the real
encap_type/encap_rcv logic, possibly adding cache misses.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Simon Horman <horms@verge.net.au>
Cc: dev@openvswitch.org
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/udp.h

@@ -267,4 +267,5 @@ extern void udp_init(void);
 extern int udp4_ufo_send_check(struct sk_buff *skb);
 extern struct sk_buff *udp4_ufo_fragment(struct sk_buff *skb,
 	netdev_features_t features);
+extern void udp_encap_enable(void);
 #endif	/* _UDP_H */

net/ipv4/udp.c

@@ -107,6 +107,7 @@
 #include <net/checksum.h>
 #include <net/xfrm.h>
 #include <trace/events/udp.h>
+#include <linux/static_key.h>
 #include "udp_impl.h"
 
 struct udp_table udp_table __read_mostly;
@@ -1379,6 +1380,14 @@ static int __udp_queue_rcv_skb(struct sock *sk, struct sk_buff *skb)
 
 }
 
+static struct static_key udp_encap_needed __read_mostly;
+void udp_encap_enable(void)
+{
+	if (!static_key_enabled(&udp_encap_needed))
+		static_key_slow_inc(&udp_encap_needed);
+}
+EXPORT_SYMBOL(udp_encap_enable);
+
 /* returns:
  *  -1: error
  *   0: success
@@ -1400,7 +1409,7 @@ int udp_queue_rcv_skb(struct sock *sk, struct sk_buff *skb)
 		goto drop;
 	nf_reset(skb);
 
-	if (up->encap_type) {
+	if (static_key_false(&udp_encap_needed) && up->encap_type) {
 		int (*encap_rcv)(struct sock *sk, struct sk_buff *skb);
 
 		/*
@@ -1760,6 +1769,7 @@ int udp_lib_setsockopt(struct sock *sk, int level, int optname,
 			/* FALLTHROUGH */
 		case UDP_ENCAP_L2TPINUDP:
 			up->encap_type = val;
+			udp_encap_enable();
 			break;
 		default:
 			err = -ENOPROTOOPT;

net/l2tp/l2tp_core.c

@@ -1424,6 +1424,7 @@ int l2tp_tunnel_create(struct net *net, int fd, int version, u32 tunnel_id, u32
 		/* Mark socket as an encapsulation socket. See net/ipv4/udp.c */
 		udp_sk(sk)->encap_type = UDP_ENCAP_L2TPINUDP;
 		udp_sk(sk)->encap_rcv = l2tp_udp_encap_recv;
+		udp_encap_enable();
 	}
 
 	sk->sk_user_data = tunnel;