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

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