linux-kernel - [PATCH 22/22] x86_64: use relative pointers with dynamic debug

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Message-Id: <20180919220444.23190-23-linux@rasmusvillemoes.dk>
Date:   Thu, 20 Sep 2018 00:04:44 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Jason Baron <jbaron@...mai.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org,
        Rasmus Villemoes <linux@...musvillemoes.dk>, x86@...nel.org
Subject: [PATCH 22/22] x86_64: use relative pointers with dynamic debug

Similar to how x86_64 uses bug_entry-relative pointers to reduce
sizeof(struct bug_entry), the same thing can now be done for struct
_ddebug, saving 16 bytes for each of those (i.e., each pr_debug, dev_dbg
etc. in a CONFIG_DYNAMIC_DEBUG kernel).

Note the use of .ifndef/.endif in asm to avoid

fs/aio.c:1382: Error: symbol `__UNIQUE_ID_ddebug112' is already defined

This is due to uses of pr_debug et al in functions that get inlined. In
such a case, __COUNTER__ is obviously only expanded once, but the asm()
is repeated once for every inlined instance. Letting all instances share
the same descriptor object is the right thing to do; that is also what
happens when it is the compiler that defines a static object inside an
inline(d) function.

Cc: x86@...nel.org
Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
---
 arch/x86/Kconfig                     |  1 +
 arch/x86/include/asm/dynamic_debug.h | 35 +++++++++++++++++++++++++++++++++++
 2 files changed, 36 insertions(+)
 create mode 100644 arch/x86/include/asm/dynamic_debug.h

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 1a0be022f91d..a44168930e52 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -90,6 +90,7 @@ config X86
 	select CLOCKSOURCE_WATCHDOG
 	select DCACHE_WORD_ACCESS
 	select DMA_DIRECT_OPS
+	select DYNAMIC_DEBUG_RELATIVE_POINTERS	if X86_64
 	select EDAC_ATOMIC_SCRUB
 	select EDAC_SUPPORT
 	select GENERIC_CLOCKEVENTS
diff --git a/arch/x86/include/asm/dynamic_debug.h b/arch/x86/include/asm/dynamic_debug.h
new file mode 100644
index 000000000000..350ef6e2feff
--- /dev/null
+++ b/arch/x86/include/asm/dynamic_debug.h
@@ -0,0 +1,35 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_X86_DYNAMIC_DEBUG_H
+#define _ASM_X86_DYNAMIC_DEBUG_H
+
+#ifdef HAVE_JUMP_LABEL
+# ifdef DEBUG
+#  define _DPRINTK_ASM_KEY_INIT ASM_STATIC_KEY_INIT_TRUE
+# else
+#  define _DPRINTK_ASM_KEY_INIT ASM_STATIC_KEY_INIT_FALSE
+# endif
+#else
+# define _DPRINTK_ASM_KEY_INIT ""
+#endif
+
+#define DEFINE_DYNAMIC_DEBUG_METADATA(name, fmt)			\
+	extern struct _ddebug name;					\
+	asm volatile(".ifndef " __stringify(name) "\n"			\
+		     ".pushsection __verbose,\"aw\"\n"			\
+		     "1:\n"						\
+		     __stringify(name) ":\n"				\
+		     "\t.long %c0 - 1b \t# _ddebug::modname_disp\n"	\
+		     "\t.long %c1 - 1b \t# _ddebug::function_disp\n"	\
+		     "\t.long %c2 - 1b \t# _ddebug::filename_disp\n"	\
+		     "\t.long %c3 - 1b \t# _ddebug::format_disp\n"	\
+		     "\t.long %c4      \t# _ddebug::flags_lineno\n"	\
+		     "\t.long 0        \t# <padding>\n"			\
+		     _DPRINTK_ASM_KEY_INIT				\
+		     ".popsection\n"					\
+		     ".endif\n"						\
+		     : : "i" (KBUILD_MODNAME), "i" (__func__),		\
+		       "i" (__FILE__), "i" (fmt),			\
+		       "i" (_DPRINTK_FLAGS_LINENO_INIT))
+
+#endif /* _ASM_X86_DYNAMIC_DEBUG_H */
+
-- 
2.16.4