linux-kernel - [PATCH 1/5 v2] x86/jump-label: Use best default nops for inital jump label calls

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Date:	Sat, 28 Jan 2012 11:09:26 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	linux-kernel@...r.kernel.org
Cc:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jason Baron <jbaron@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"H. Peter Anvin" <hpa@...ux.intel.com>
Subject: [PATCH 1/5 v2] x86/jump-label: Use best default nops for inital jump label calls

From: Steven Rostedt <srostedt@...hat.com>

As specified by H. Peter Anvin, the best nops for x86 without knowing
the running computer is:

32bit:
  0x3e, 0x8d, 0x74, 0x26, 0x00 also known as GENERIC_NOP5_ATOMIC

64bit:
  0x0f, 0x1f, 0x44, 0x00, 0x00  also known as P6_NOP5_ATOMIC

Currently the default nop that is used by jump label is:

 0xe9 0x00 0x00 0x00 0x00

Which is really a 5byte jump to the next position.

It's better to use a real nop than a jmp.

Cc: H. Peter Anvin <hpa@...ux.intel.com>
Cc: Jason Baron <jbaron@...hat.com>
Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
---
 arch/x86/include/asm/jump_label.h |    9 +++++++--
 1 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/jump_label.h b/arch/x86/include/asm/jump_label.h
index a32b18c..7082743 100644
--- a/arch/x86/include/asm/jump_label.h
+++ b/arch/x86/include/asm/jump_label.h
@@ -3,18 +3,23 @@
 
 #ifdef __KERNEL__
 
+#include <linux/stringify.h>
 #include <linux/types.h>
 #include <asm/nops.h>
 #include <asm/asm.h>
 
 #define JUMP_LABEL_NOP_SIZE 5
 
-#define JUMP_LABEL_INITIAL_NOP ".byte 0xe9 \n\t .long 0\n\t"
+#ifdef CONFIG_X86_64
+# define JUMP_LABEL_INIT_NOP P6_NOP5_ATOMIC
+#else
+# define JUMP_LABEL_INIT_NOP GENERIC_NOP5_ATOMIC
+#endif
 
 static __always_inline bool arch_static_branch(struct jump_label_key *key)
 {
 	asm goto("1:"
-		JUMP_LABEL_INITIAL_NOP
+		".byte " __stringify(JUMP_LABEL_INIT_NOP) "\n\t"
 		".pushsection __jump_table,  \"aw\" \n\t"
 		_ASM_ALIGN "\n\t"
 		_ASM_PTR "1b, %l[l_yes], %c0 \n\t"
-- 
1.7.8.3



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