[tip:x86/asm] x86-64, copy_user: Remove zero byte check before copy user buffer.
From: tip-bot for Fenghua Yu
Date: Sun Nov 17 2013 - 01:19:49 EST
Commit-ID: f4cb1cc18f364d761d5614eb6293cccc6647f259
Gitweb: http://git.kernel.org/tip/f4cb1cc18f364d761d5614eb6293cccc6647f259
Author: Fenghua Yu <fenghua.yu@xxxxxxxxx>
AuthorDate: Sat, 16 Nov 2013 12:37:01 -0800
Committer: H. Peter Anvin <hpa@xxxxxxxxxxxxxxx>
CommitDate: Sat, 16 Nov 2013 18:00:58 -0800
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@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Fenghua Yu <fenghua.yu@xxxxxxxxx>
Link: http://lkml.kernel.org/r/1384634221-6006-1-git-send-email-fenghua.yu@xxxxxxxxx
Signed-off-by: H. Peter Anvin <hpa@xxxxxxxxxxxxxxx>
---
arch/x86/lib/copy_user_64.S | 8 ++------
1 file changed, 2 insertions(+), 6 deletions(-)
diff --git a/arch/x86/lib/copy_user_64.S b/arch/x86/lib/copy_user_64.S
index a30ca15..ffe4eb9 100644
--- a/arch/x86/lib/copy_user_64.S
+++ b/arch/x86/lib/copy_user_64.S
@@ -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
--
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