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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) ENTRY(copy_user_generic_string)
CFI_STARTPROC CFI_STARTPROC
ASM_STAC ASM_STAC
andl %edx,%edx
jz 4f
cmpl $8,%edx cmpl $8,%edx
jb 2f /* less than 8 bytes, go to byte copy loop */ jb 2f /* less than 8 bytes, go to byte copy loop */
ALIGN_DESTINATION ALIGN_DESTINATION
@ -249,7 +247,7 @@ ENTRY(copy_user_generic_string)
2: movl %edx,%ecx 2: movl %edx,%ecx
3: rep 3: rep
movsb movsb
4: xorl %eax,%eax xorl %eax,%eax
ASM_CLAC ASM_CLAC
ret ret
@ -279,12 +277,10 @@ ENDPROC(copy_user_generic_string)
ENTRY(copy_user_enhanced_fast_string) ENTRY(copy_user_enhanced_fast_string)
CFI_STARTPROC CFI_STARTPROC
ASM_STAC ASM_STAC
andl %edx,%edx
jz 2f
movl %edx,%ecx movl %edx,%ecx
1: rep 1: rep
movsb movsb
2: xorl %eax,%eax xorl %eax,%eax
ASM_CLAC ASM_CLAC
ret ret