netfilter: nf_tables: add static key to skip retpoline workarounds

If CONFIG_RETPOLINE is enabled nf_tables avoids indirect calls for
builtin expressions.

On newer cpus indirect calls do not go through the retpoline thunk
anymore, even for RETPOLINE=y builds.

Just like with the new tc retpoline wrappers:
Add a static key to skip the if / else if cascade if the cpu
does not require retpolines.
Suggested-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>

net/netfilter/nf_tables_core.c

@@ -21,6 +21,26 @@
 #include <net/netfilter/nf_log.h>
 #include <net/netfilter/nft_meta.h>
 
+#if defined(CONFIG_RETPOLINE) && defined(CONFIG_X86)
+
+static struct static_key_false nf_tables_skip_direct_calls;
+
+static bool nf_skip_indirect_calls(void)
+{
+	return static_branch_likely(&nf_tables_skip_direct_calls);
+}
+
+static void __init nf_skip_indirect_calls_enable(void)
+{
+	if (!cpu_feature_enabled(X86_FEATURE_RETPOLINE))
+		static_branch_enable(&nf_tables_skip_direct_calls);
+}
+#else
+static inline bool nf_skip_indirect_calls(void) { return false; }
+
+static inline void nf_skip_indirect_calls_enable(void) { }
+#endif
+
 static noinline void __nft_trace_packet(struct nft_traceinfo *info,
 					const struct nft_chain *chain,
 					enum nft_trace_types type)
@@ -193,7 +213,12 @@ static void expr_call_ops_eval(const struct nft_expr *expr,
 			       struct nft_pktinfo *pkt)
 {
 #ifdef CONFIG_RETPOLINE
-	unsigned long e = (unsigned long)expr->ops->eval;
+	unsigned long e;
+
+	if (nf_skip_indirect_calls())
+		goto indirect_call;
+
+	e = (unsigned long)expr->ops->eval;
 #define X(e, fun) \
 	do { if ((e) == (unsigned long)(fun)) \
 		return fun(expr, regs, pkt); } while (0)
@@ -210,6 +235,7 @@ static void expr_call_ops_eval(const struct nft_expr *expr,
 	X(e, nft_rt_get_eval);
 	X(e, nft_bitwise_eval);
 #undef  X
+indirect_call:
 #endif /* CONFIG_RETPOLINE */
 	expr->ops->eval(expr, regs, pkt);
 }
@@ -369,6 +395,8 @@ int __init nf_tables_core_module_init(void)
 			goto err;
 	}
 
+	nf_skip_indirect_calls_enable();
+
 	return 0;
 
 err: