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>

2 files changed, 12 insertions, 1 deletions

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index fe141052a1be..ad1e0dd4da48 100644
--- a/net/ipv4/udp.c
+++ b/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;
diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index 89ff8c67943e..f6732b6c758b 100644
--- a/net/l2tp/l2tp_core.c
+++ b/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;