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x86-64, copy_user: Remove zero byte check before copy user buffer.

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Operation of rep movsb instruction handles zero byte copy. As pointed out by
Linus, there is no need to check zero size in kernel. Removing this redundant
check saves a few cycles in copy user functions.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Link: http://lkml.kernel.org/r/1384634221-6006-1-git-send-email-fenghua.yu@intel.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
This commit is contained in:
Fenghua Yu 2013-11-16 12:37:01 -08:00 • committed by H. Peter Anvin
parent 1213959d4a
commit f4cb1cc18f
1 changed files with 2 additions and 6 deletions
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8
arch/x86/lib/copy_user_64.S
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@ -236,8 +236,6 @@ ENDPROC(copy_user_generic_unrolled)
ENTRY(copy_user_generic_string)
CFI_STARTPROC
ASM_STAC
andl %edx,%edx
jz 4f
cmpl $8,%edx
jb 2f /* less than 8 bytes, go to byte copy loop */
ALIGN_DESTINATION
@ -249,7 +247,7 @@ ENTRY(copy_user_generic_string)
2: movl %edx,%ecx
3: rep
movsb
4: xorl %eax,%eax
xorl %eax,%eax
ASM_CLAC
ret
@ -279,12 +277,10 @@ ENDPROC(copy_user_generic_string)
ENTRY(copy_user_enhanced_fast_string)
CFI_STARTPROC
ASM_STAC
andl %edx,%edx
jz 2f
movl %edx,%ecx
1: rep
movsb
2: xorl %eax,%eax
xorl %eax,%eax
ASM_CLAC
ret