tcp: md5: add tcp_md5_needed jump label

Most linux hosts never setup TCP MD5 keys. We can avoid a
cache line miss (accessing tp->md5ig_info) on RX and TX
using a jump label.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -1555,9 +1555,21 @@ struct tcp_md5sig_key *tcp_v4_md5_lookup(const struct sock *sk,
 					 const struct sock *addr_sk);
 
 #ifdef CONFIG_TCP_MD5SIG
-struct tcp_md5sig_key *tcp_md5_do_lookup(const struct sock *sk,
-					 const union tcp_md5_addr *addr,
-					 int family);
+#include <linux/jump_label.h>
+extern struct static_key tcp_md5_needed;
+struct tcp_md5sig_key *__tcp_md5_do_lookup(const struct sock *sk,
+					   const union tcp_md5_addr *addr,
+					   int family);
+static inline struct tcp_md5sig_key *
+tcp_md5_do_lookup(const struct sock *sk,
+		  const union tcp_md5_addr *addr,
+		  int family)
+{
+	if (!static_key_false(&tcp_md5_needed))
+		return NULL;
+	return __tcp_md5_do_lookup(sk, addr, family);
+}
+
 #define tcp_twsk_md5_key(twsk)	((twsk)->tw_md5_key)
 #else
 static inline struct tcp_md5sig_key *tcp_md5_do_lookup(const struct sock *sk,

net/ipv4/tcp.c

@@ -3656,8 +3656,11 @@ bool tcp_alloc_md5sig_pool(void)
 	if (unlikely(!tcp_md5sig_pool_populated)) {
 		mutex_lock(&tcp_md5sig_mutex);
 
-		if (!tcp_md5sig_pool_populated)
+		if (!tcp_md5sig_pool_populated) {
 			__tcp_alloc_md5sig_pool();
+			if (tcp_md5sig_pool_populated)
+				static_key_slow_inc(&tcp_md5_needed);
+		}
 
 		mutex_unlock(&tcp_md5sig_mutex);
 	}

net/ipv4/tcp_ipv4.c

@@ -970,10 +970,13 @@ static void tcp_v4_reqsk_destructor(struct request_sock *req)
  * We need to maintain these in the sk structure.
  */
 
+struct static_key tcp_md5_needed __read_mostly;
+EXPORT_SYMBOL(tcp_md5_needed);
+
 /* Find the Key structure for an address.  */
-struct tcp_md5sig_key *tcp_md5_do_lookup(const struct sock *sk,
-					 const union tcp_md5_addr *addr,
-					 int family)
+struct tcp_md5sig_key *__tcp_md5_do_lookup(const struct sock *sk,
+					   const union tcp_md5_addr *addr,
+					   int family)
 {
 	const struct tcp_sock *tp = tcp_sk(sk);
 	struct tcp_md5sig_key *key;
@@ -1011,7 +1014,7 @@ struct tcp_md5sig_key *tcp_md5_do_lookup(const struct sock *sk,
 	}
 	return best_match;
 }
-EXPORT_SYMBOL(tcp_md5_do_lookup);
+EXPORT_SYMBOL(__tcp_md5_do_lookup);
 
 static struct tcp_md5sig_key *tcp_md5_do_lookup_exact(const struct sock *sk,
 						      const union tcp_md5_addr *addr,

net/ipv4/tcp_output.c

@@ -594,7 +594,8 @@ static unsigned int tcp_syn_options(struct sock *sk, struct sk_buff *skb,
 
 	*md5 = NULL;
 #ifdef CONFIG_TCP_MD5SIG
-	if (unlikely(rcu_access_pointer(tp->md5sig_info))) {
+	if (static_key_false(&tcp_md5_needed) &&
+	    rcu_access_pointer(tp->md5sig_info)) {
 		*md5 = tp->af_specific->md5_lookup(sk, sk);
 		if (*md5) {
 			opts->options |= OPTION_MD5;
@@ -730,7 +731,8 @@ static unsigned int tcp_established_options(struct sock *sk, struct sk_buff *skb
 
 	*md5 = NULL;
 #ifdef CONFIG_TCP_MD5SIG
-	if (unlikely(rcu_access_pointer(tp->md5sig_info))) {
+	if (static_key_false(&tcp_md5_needed) &&
+	    rcu_access_pointer(tp->md5sig_info)) {
 		*md5 = tp->af_specific->md5_lookup(sk, sk);
 		if (*md5) {
 			opts->options |= OPTION_MD5;