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From: Nadav Amit <namit@vmware.com>
To: Ingo Molnar <mingo@redhat.com>
Cc: <x86@kernel.org>, Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	<linux-kernel@vger.kernel.org>, Nadav Amit <namit@vmware.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Josh Poimboeuf <jpoimboe@redhat.com>
Subject: [PATCH v7 06/10] x86: bug: prevent gcc distortions
Date: Thu, 9 Aug 2018 13:15:49 -0700
Message-ID: <20180809201554.168804-7-namit@vmware.com> (raw)
In-Reply-To: <20180809201554.168804-1-namit@vmware.com>

GCC considers the number of statements in inlined assembly blocks,
according to new-lines and semicolons, as an indication to the cost of
the block in time and space. This data is distorted by the kernel code,
which puts information in alternative sections. As a result, the
compiler may perform incorrect inlining and branch optimizations.

The solution is to set an assembly macro and call it from the inlinedv
assembly block. As a result GCC considers the inline assembly block as
a single instruction.

This patch increases the kernel size:

   text	   data	    bss	    dec	    hex	filename
18146889 10225380 2957312 31329581 1de0d2d ./vmlinux before
18147336 10226688 2957312 31331336 1de1408 ./vmlinux after (+1755)

But enables more aggressive inlining (and probably branch decisions).
The number of static text symbols in vmlinux is lower.

Static text symbols:
Before:	40218
After:	40053	(-165)

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Nadav Amit <namit@vmware.com>
---
 arch/x86/include/asm/bug.h | 98 ++++++++++++++++++++++----------------
 arch/x86/kernel/macros.S   |  1 +
 include/asm-generic/bug.h  |  8 ++--
 3 files changed, 61 insertions(+), 46 deletions(-)

diff --git a/arch/x86/include/asm/bug.h b/arch/x86/include/asm/bug.h
index 6804d6642767..5090035e6d16 100644
--- a/arch/x86/include/asm/bug.h
+++ b/arch/x86/include/asm/bug.h
@@ -4,6 +4,8 @@
 
 #include <linux/stringify.h>
 
+#ifndef __ASSEMBLY__
+
 /*
  * Despite that some emulators terminate on UD2, we use it for WARN().
  *
@@ -20,53 +22,15 @@
 
 #define LEN_UD2		2
 
-#ifdef CONFIG_GENERIC_BUG
-
-#ifdef CONFIG_X86_32
-# define __BUG_REL(val)	".long " __stringify(val)
-#else
-# define __BUG_REL(val)	".long " __stringify(val) " - 2b"
-#endif
-
-#ifdef CONFIG_DEBUG_BUGVERBOSE
-
-#define _BUG_FLAGS(ins, flags)						\
-do {									\
-	asm volatile("1:\t" ins "\n"					\
-		     ".pushsection __bug_table,\"aw\"\n"		\
-		     "2:\t" __BUG_REL(1b) "\t# bug_entry::bug_addr\n"	\
-		     "\t"  __BUG_REL(%c0) "\t# bug_entry::file\n"	\
-		     "\t.word %c1"        "\t# bug_entry::line\n"	\
-		     "\t.word %c2"        "\t# bug_entry::flags\n"	\
-		     "\t.org 2b+%c3\n"					\
-		     ".popsection"					\
-		     : : "i" (__FILE__), "i" (__LINE__),		\
-			 "i" (flags),					\
-			 "i" (sizeof(struct bug_entry)));		\
-} while (0)
-
-#else /* !CONFIG_DEBUG_BUGVERBOSE */
-
 #define _BUG_FLAGS(ins, flags)						\
 do {									\
-	asm volatile("1:\t" ins "\n"					\
-		     ".pushsection __bug_table,\"aw\"\n"		\
-		     "2:\t" __BUG_REL(1b) "\t# bug_entry::bug_addr\n"	\
-		     "\t.word %c0"        "\t# bug_entry::flags\n"	\
-		     "\t.org 2b+%c1\n"					\
-		     ".popsection"					\
-		     : : "i" (flags),					\
+	asm volatile("ASM_BUG ins=\"" ins "\" file=%c0 line=%c1 "	\
+		     "flags=%c2 size=%c3"				\
+		     : : "i" (__FILE__), "i" (__LINE__),                \
+			 "i" (flags),                                   \
 			 "i" (sizeof(struct bug_entry)));		\
 } while (0)
 
-#endif /* CONFIG_DEBUG_BUGVERBOSE */
-
-#else
-
-#define _BUG_FLAGS(ins, flags)  asm volatile(ins)
-
-#endif /* CONFIG_GENERIC_BUG */
-
 #define HAVE_ARCH_BUG
 #define BUG()							\
 do {								\
@@ -82,4 +46,54 @@ do {								\
 
 #include <asm-generic/bug.h>
 
+#else /* __ASSEMBLY__ */
+
+#ifdef CONFIG_GENERIC_BUG
+
+#ifdef CONFIG_X86_32
+.macro __BUG_REL val:req
+	.long \val
+.endm
+#else
+.macro __BUG_REL val:req
+	.long \val - 2b
+.endm
+#endif
+
+#ifdef CONFIG_DEBUG_BUGVERBOSE
+
+.macro ASM_BUG ins:req file:req line:req flags:req size:req
+1:	\ins
+	.pushsection __bug_table,"aw"
+2:	__BUG_REL val=1b	# bug_entry::bug_addr
+	__BUG_REL val=\file	# bug_entry::file
+	.word \line		# bug_entry::line
+	.word \flags		# bug_entry::flags
+	.org 2b+\size
+	.popsection
+.endm
+
+#else /* !CONFIG_DEBUG_BUGVERBOSE */
+
+.macro ASM_BUG ins:req file:req line:req flags:req size:req
+1:	\ins
+	.pushsection __bug_table,"aw"
+2:	__BUG_REL val=1b	# bug_entry::bug_addr
+	.word \flags		# bug_entry::flags
+	.org 2b+\size
+	.popsection
+.endm
+
+#endif /* CONFIG_DEBUG_BUGVERBOSE */
+
+#else /* CONFIG_GENERIC_BUG */
+
+.macro ASM_BUG ins:req file:req line:req flags:req size:req
+	\ins
+.endm
+
+#endif /* CONFIG_GENERIC_BUG */
+
+#endif /* __ASSEMBLY__ */
+
 #endif /* _ASM_X86_BUG_H */
diff --git a/arch/x86/kernel/macros.S b/arch/x86/kernel/macros.S
index 852487a9fc56..66ccb8e823b1 100644
--- a/arch/x86/kernel/macros.S
+++ b/arch/x86/kernel/macros.S
@@ -9,3 +9,4 @@
 #include <linux/compiler.h>
 #include <asm/refcount.h>
 #include <asm/alternative-asm.h>
+#include <asm/bug.h>
diff --git a/include/asm-generic/bug.h b/include/asm-generic/bug.h
index a7613e1b0c87..cf7f4ef4749e 100644
--- a/include/asm-generic/bug.h
+++ b/include/asm-generic/bug.h
@@ -17,10 +17,8 @@
 #ifndef __ASSEMBLY__
 #include <linux/kernel.h>
 
-#ifdef CONFIG_BUG
-
-#ifdef CONFIG_GENERIC_BUG
 struct bug_entry {
+#ifdef CONFIG_GENERIC_BUG
 #ifndef CONFIG_GENERIC_BUG_RELATIVE_POINTERS
 	unsigned long	bug_addr;
 #else
@@ -35,8 +33,10 @@ struct bug_entry {
 	unsigned short	line;
 #endif
 	unsigned short	flags;
-};
 #endif	/* CONFIG_GENERIC_BUG */
+};
+
+#ifdef CONFIG_BUG
 
 /*
  * Don't use BUG() or BUG_ON() unless there's really no way out; one
-- 
2.17.1


  parent reply index

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-08-09 20:15 [PATCH v7 00/10] x86: macrofying inline asm for better compilation Nadav Amit
2018-08-09 20:15 ` [PATCH v7 01/10] xtensa: defining LINKER_SCRIPT for the linker script Nadav Amit
2018-08-09 20:25   ` Max Filippov
2018-08-09 20:15 ` [PATCH v7 02/10] Makefile: Prepare for using macros for inline asm Nadav Amit
2018-08-10  1:39   ` Masahiro Yamada
2018-08-09 20:15 ` [PATCH v7 03/10] x86: objtool: use asm macro for better compiler decisions Nadav Amit
2018-08-09 20:15 ` [PATCH v7 04/10] x86: refcount: prevent gcc distortions Nadav Amit
2018-08-09 20:15 ` [PATCH v7 05/10] x86: alternatives: macrofy locks for better inlining Nadav Amit
2018-08-09 20:15 ` Nadav Amit [this message]
2018-08-09 20:15 ` [PATCH v7 07/10] x86: prevent inline distortion by paravirt ops Nadav Amit
2018-08-09 20:15 ` [PATCH v7 08/10] x86: extable: use macros instead of inline assembly Nadav Amit
2018-08-09 20:15 ` [PATCH v7 09/10] x86: cpufeature: " Nadav Amit
2018-08-09 20:15 ` [PATCH v7 10/10] x86: jump-labels: " Nadav Amit
2018-09-04 17:14 ` [PATCH v7 00/10] x86: macrofying inline asm for better compilation Nadav Amit
2018-09-10 13:00   ` Ingo Molnar
2018-09-10 17:00     ` Nadav Amit

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