[PATCH 1/4] x86/jump-label: Use best default nops for inital jump label calls

Steven Rostedt
2013-08-07T16:52:17Z

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From: Steven Rostedt <srostedt@redhat.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@linux.intel.com>
Cc: Jason Baron <jbaron@redhat.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
arch/x86/include/asm/jump_label.h | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/jump_label.h b/arch/x86/include/asm/jump_label.h
index 3a16c14..64507f3 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 STATIC_KEY_INITIAL_NOP ".byte 0xe9 \n\t .long 0\n\t"
+#ifdef CONFIG_X86_64
+# define STATIC_KEY_INIT_NOP P6_NOP5_ATOMIC
+#else
+# define STATIC_KEY_INIT_NOP GENERIC_NOP5_ATOMIC
+#endif

static __always_inline bool arch_static_branch(struct static_key *key)
{
asm goto("1:"
- STATIC_KEY_INITIAL_NOP
+ ".byte " __stringify(STATIC_KEY_INIT_NOP) "\n\t"
".pushsection __jump_table, \"aw\" \n\t"
_ASM_ALIGN "\n\t"
_ASM_PTR "1b, %l[l_yes], %c0 \n\t"
--
1.7.10.4