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Commit f4cb1cc1 authored by Fenghua Yu's avatar Fenghua Yu Committed by H. Peter Anvin
Browse files

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: default avatarLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: default avatarFenghua Yu <fenghua.yu@intel.com>
Link: http://lkml.kernel.org/r/1384634221-6006-1-git-send-email-fenghua.yu@intel.com


Signed-off-by: default avatarH. Peter Anvin <hpa@linux.intel.com>
parent 1213959d

arch/x86/lib/copy_user_64.S

+2 −6
Original line number Diff line number Diff line
@@ -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