netdev - [RFC PATCH 1/4] indirect call wrappers: helpers to speed-up indirect calls of builtin

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Date:   Fri, 30 Nov 2018 00:00:11 +0100
From:   Paolo Abeni <pabeni@...hat.com>
To:     netdev@...r.kernel.org
Cc:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: [RFC PATCH 1/4] indirect call wrappers: helpers to speed-up indirect calls of builtin

This header define a bunch of helpers that allow avoiding the
retpoline overhead when calling builtin functions via function pointers.
It boils down to explicitly comparing the function pointers to
known builtin functions and eventually invoke directly the latter.

The macro defined here implement the boilerplate for the above schema
and will be used by the next patches.

Suggested-by: Eric Dumazet <Eric Dumazet <edumazet@...gle.com>
Signed-off-by: Paolo Abeni <pabeni@...hat.com>
---
 include/linux/indirect_call_wrapper.h | 77 +++++++++++++++++++++++++++
 1 file changed, 77 insertions(+)
 create mode 100644 include/linux/indirect_call_wrapper.h

diff --git a/include/linux/indirect_call_wrapper.h b/include/linux/indirect_call_wrapper.h
new file mode 100644
index 000000000000..57e82b4a166d
--- /dev/null
+++ b/include/linux/indirect_call_wrapper.h
@@ -0,0 +1,77 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_INDIRECT_CALL_WRAPPER_H
+#define _LINUX_INDIRECT_CALL_WRAPPER_H
+
+#ifdef CONFIG_RETPOLINE
+
+/*
+ * INDIRECT_CALL_$NR - wrapper for indirect calls with $NR known builtin
+ *  @f: function pointer
+ *  @name: base name for builtin functions, see INDIRECT_CALLABLE_DECLARE_$NR
+ *  @__VA_ARGS__: arguments for @f
+ *
+ * Avoid retpoline overhead for known builtin, checking @f vs each of them and
+ * eventually invoking directly the builtin function. Fallback to the indirect
+ * call
+ */
+#define INDIRECT_CALL_1(f, name, ...)					\
+	({								\
+		f == name ## 1 ? name ## 1(__VA_ARGS__) :		\
+				 f(__VA_ARGS__);			\
+	})
+#define INDIRECT_CALL_2(f, name, ...)					\
+	({								\
+		f == name ## 2 ? name ## 2(__VA_ARGS__) :		\
+				 INDIRECT_CALL_1(f, name, __VA_ARGS__);	\
+	})
+
+/*
+ * INDIRECT_CALLABLE_DECLARE_$NR - declare $NR known builtin for
+ * INDIRECT_CALL_$NR usage
+ *  @type: return type for the builtin function
+ *  @name: base name for builtin functions, the full list is generated appending
+ *	   the numbers in the 1..@NR range
+ *  @__VA_ARGS__: arguments type list for the builtin function
+ *
+ * Builtin with higher $NR will be checked first by INDIRECT_CALL_$NR
+ */
+#define INDIRECT_CALLABLE_DECLARE_1(type, name, ...)			\
+	extern type name ## 1(__VA_ARGS__)
+#define INDIRECT_CALLABLE_DECLARE_2(type, name, ...)			\
+	extern type name ## 2(__VA_ARGS__);				\
+	INDIRECT_CALLABLE_DECLARE_1(type, name, __VA_ARGS__)
+
+/*
+ * INDIRECT_CALLABLE - allow usage of a builtin function from INDIRECT_CALL_$NR
+ *  @f: builtin function name
+ *  @nr: id associated with this builtin, higher values will be checked first by
+ *	 INDIRECT_CALL_$NR
+ *  @type: function return type
+ *  @name: base name used by INDIRECT_CALL_ to access the builtin list
+ *  @__VA_ARGS__: arguments type list for @f
+ */
+#define INDIRECT_CALLABLE(f, nr, type, name, ...)		\
+	__alias(f) type name ## nr(__VA_ARGS__)
+
+#else
+#define INDIRECT_CALL_1(f, name, ...) f(__VA_ARGS__)
+#define INDIRECT_CALL_2(f, name, ...) f(__VA_ARGS__)
+#define INDIRECT_CALLABLE_DECLARE_1(type, name, ...)
+#define INDIRECT_CALLABLE_DECLARE_2(type, name, ...)
+#define INDIRECT_CALLABLE(f, nr, type, name, ...)
+#endif
+
+/*
+ * We can use INDIRECT_CALL_$NR for ipv6 related functions only if ipv6 is
+ * builtin, this macro simplify dealing with indirect calls with only ipv4/ipv6
+ * alternatives
+ */
+#if IS_BUILTIN(CONFIG_IPV6)
+#define INDIRECT_CALL_INET INDIRECT_CALL_2
+#elif IS_ENABLED(CONFIG_INET)
+#define INDIRECT_CALL_INET INDIRECT_CALL_1
+#else
+#define INDIRECT_CALL_INET(...)
+#endif
+
+#endif
-- 
2.19.2