linux-kernel - [PATCH v3 23/23] x86_64: use relative pointers with dynamic debug

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Message-Id: <20181109231021.11658-24-linux@rasmusvillemoes.dk>
Date:   Sat, 10 Nov 2018 00:10:21 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Jason Baron <jbaron@...mai.com>
Cc:     linux-kernel@...r.kernel.org,
        Rasmus Villemoes <linux@...musvillemoes.dk>, x86@...nel.org,
        Ingo Molnar <mingo@...nel.org>
Subject: [PATCH v3 23/23] 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.

That, however, leads to another problem if the same identifier is used
in two different DEFINE_DYNAMIC_DEBUG_METADATA() in the same translation
unit: They would both end up referring to the same (first occuring)
instance. So the second one would end up using the wrong file, line,
function etc. information. All current in-tree macro users of
DEFINE_DYNAMIC_DEBUG_METADATA() have been updated to ensure they use
UNIQUE_ID to generate the identifier.

To prevent such a problem from going unnoticed, we use a guard symbol in
assembly whose value is unique per DEFINE_DYNAMIC_DEBUG_METADATA
invocation.

Cc: x86@...nel.org
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Jason Baron <jbaron@...mai.com>
Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
---
 arch/x86/Kconfig                     |  1 +
 arch/x86/include/asm/dynamic_debug.h | 58 ++++++++++++++++++++++++++++
 arch/x86/kernel/macros.S             |  1 +
 3 files changed, 60 insertions(+)
 create mode 100644 arch/x86/include/asm/dynamic_debug.h

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index ba7e3464ee92..c7cb31f39e5a 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -91,6 +91,7 @@ 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..80730409183f
--- /dev/null
+++ b/arch/x86/include/asm/dynamic_debug.h
@@ -0,0 +1,58 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_X86_DYNAMIC_DEBUG_H
+#define _ASM_X86_DYNAMIC_DEBUG_H
+
+#ifndef __ASSEMBLY__
+
+#ifdef DEBUG
+#define _DPRINTK_INIT_TRUE 1
+#else
+#define _DPRINTK_INIT_TRUE 0
+#endif
+
+#define DEFINE_DYNAMIC_DEBUG_METADATA(name, fmt)			\
+	extern struct _ddebug name;					\
+	asm volatile("DYNAMIC_DEBUG_METADATA"				\
+		     " ident="__stringify(name)				\
+		     " modname=%c0"					\
+		     " func=%c1"					\
+		     " file=%c2"					\
+		     " fmt=%c3"						\
+		     " flag_line=%c4"					\
+		     " init_true=%c5"					\
+		     " cnt=%c6"						\
+		     : : "i" (KBUILD_MODNAME), "i" (__func__),		\
+		       "i" (__FILE__), "i" (fmt),			\
+		       "i" (_DPRINTK_FLAGS_LINENO_INIT),		\
+		       "i" (_DPRINTK_INIT_TRUE),			\
+		       "i" (__COUNTER__)				\
+		)
+
+#else	/* __ASSEMBLY__ */
+
+.macro DYNAMIC_DEBUG_METADATA ident modname func file fmt flag_line init_true cnt
+.ifndef \ident
+	.pushsection __verbose,"aw"
+\ident :
+	.long \modname - \ident
+	.long \func    - \ident
+	.long \file    - \ident
+	.long \fmt     - \ident
+	.long \flag_line
+	.long 0
+	.if \init_true
+	STATIC_KEY_INIT_TRUE
+	.else
+	STATIC_KEY_INIT_FALSE
+	.endif
+	.popsection
+	.set \ident\().ddebug.once, \cnt
+.elseif \ident\().ddebug.once - \cnt
+	.error "'\ident' used as _ddebug identifier more than once"
+.endif
+.endm
+
+#endif	/* __ASSEMBLY__ */
+
+#endif /* _ASM_X86_DYNAMIC_DEBUG_H */
+
diff --git a/arch/x86/kernel/macros.S b/arch/x86/kernel/macros.S
index 161c95059044..2c2b8e5229a9 100644
--- a/arch/x86/kernel/macros.S
+++ b/arch/x86/kernel/macros.S
@@ -14,3 +14,4 @@
 #include <asm/asm.h>
 #include <asm/cpufeature.h>
 #include <asm/jump_label.h>
+#include <asm/dynamic_debug.h>
-- 
2.19.1.6.gbde171bbf5