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x86/jump-label: Use best default nops for inital jump label calls 

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>
This commit is contained in:
Steven Rostedt 2012-01-26 18:06:45 -05:00 committed by Steven Rostedt
parent ad81f0545e
commit c3c7f14a11
1 changed files with 7 additions and 2 deletions
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9
arch/x86/include/asm/jump_label.h
View File

@ -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"