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SubjectRe: [PATCH] x86: Optimize tail handling for copy_user
Date
Linus Torvalds <torvalds@linux-foundation.org> writes:

> On Mon, 28 Jul 2008, Vitaly Mayatskikh wrote:
>>
>> Reduce protection faults count in copy_user_handle_tail routine by
>> limiting clear length to the end of page as was suggested by Linus.
>
> No, you did it wrong.

Pass direction and clear remainder flags to tail handling routine.
Fixed typos with using register r8d instead of ecx. Testl instruction
used for zero checks instead of andl as it generates more effective
microcode.

Signed-off-by: Vitaly Mayatskikh <v.mayatskih@gmail.com>

diff --git a/arch/x86/lib/copy_user_64.S b/arch/x86/lib/copy_user_64.S
index dfdf428..4fbd9d5 100644
--- a/arch/x86/lib/copy_user_64.S
+++ b/arch/x86/lib/copy_user_64.S
@@ -9,12 +9,14 @@
#include <linux/linkage.h>
#include <asm/dwarf2.h>

+/* Align destination */
#define FIX_ALIGNMENT 1

#include <asm/current.h>
#include <asm/asm-offsets.h>
#include <asm/thread_info.h>
#include <asm/cpufeature.h>
+#include <asm/uaccess_64.h>

.macro ALTERNATIVE_JUMP feature,orig,alt
0:
@@ -44,15 +46,16 @@
subl $8,%ecx
negl %ecx
subl %ecx,%edx
-100: movb (%rsi),%al
-101: movb %al,(%rdi)
+100: movb (%rsi),%ah /* al is flags */
+101: movb %ah,(%rdi)
incq %rsi
incq %rdi
decl %ecx
jnz 100b
102:
.section .fixup,"ax"
-103: addl %r8d,%edx /* ecx is zerorest also */
+103: addl %ecx,%edx
+ movl %eax,%ecx
jmp copy_user_handle_tail
.previous

@@ -61,7 +64,7 @@
.quad 100b,103b
.quad 101b,103b
.previous
-#endif
+#endif /* FIX_ALIGNMENT */
.endm

/* Standard copy_to_user with segment limit checking */
@@ -73,6 +76,7 @@ ENTRY(copy_to_user)
jc bad_to_user
cmpq TI_addr_limit(%rax),%rcx
jae bad_to_user
+ movl $(DEST_IS_USERSPACE),%ecx
ALTERNATIVE_JUMP X86_FEATURE_REP_GOOD,copy_user_generic_unrolled,copy_user_generic_string
CFI_ENDPROC

@@ -85,18 +89,21 @@ ENTRY(copy_from_user)
jc bad_from_user
cmpq TI_addr_limit(%rax),%rcx
jae bad_from_user
+ movl $(SOURCE_IS_USERSPACE | CLEAR_REMAINDER),%ecx
ALTERNATIVE_JUMP X86_FEATURE_REP_GOOD,copy_user_generic_unrolled,copy_user_generic_string
CFI_ENDPROC
ENDPROC(copy_from_user)

ENTRY(copy_user_generic)
CFI_STARTPROC
+ movl $(SOURCE_IS_USERSPACE | DEST_IS_USERSPACE),%ecx
ALTERNATIVE_JUMP X86_FEATURE_REP_GOOD,copy_user_generic_unrolled,copy_user_generic_string
CFI_ENDPROC
ENDPROC(copy_user_generic)

ENTRY(__copy_from_user_inatomic)
CFI_STARTPROC
+ movl $(SOURCE_IS_USERSPACE),%ecx
ALTERNATIVE_JUMP X86_FEATURE_REP_GOOD,copy_user_generic_unrolled,copy_user_generic_string
CFI_ENDPROC
ENDPROC(__copy_from_user_inatomic)
@@ -125,13 +132,15 @@ ENDPROC(bad_from_user)
* Input:
* rdi destination
* rsi source
- * rdx count
+ * edx count
+ * ecx flags
*
* Output:
* eax uncopied bytes or 0 if successfull.
*/
ENTRY(copy_user_generic_unrolled)
CFI_STARTPROC
+ movl %ecx,%eax /* save flags in eax */
cmpl $8,%edx
jb 20f /* less then 8 bytes, go to byte copy loop */
ALIGN_DESTINATION
@@ -169,11 +178,11 @@ ENTRY(copy_user_generic_unrolled)
leaq 8(%rdi),%rdi
decl %ecx
jnz 18b
-20: andl %edx,%edx
+20: testl %edx,%edx
jz 23f
movl %edx,%ecx
-21: movb (%rsi),%al
-22: movb %al,(%rdi)
+21: movb (%rsi),%ah /* al is flags */
+22: movb %ah,(%rdi)
incq %rsi
incq %rdi
decl %ecx
@@ -188,7 +197,8 @@ ENTRY(copy_user_generic_unrolled)
40: lea (%rdx,%rcx,8),%rdx
jmp 60f
50: movl %ecx,%edx
-60: jmp copy_user_handle_tail /* ecx is zerorest also */
+60: movl %eax,%ecx /* get flags back to ecx*/
+ jmp copy_user_handle_tail
.previous

.section __ex_table,"a"
@@ -230,15 +240,15 @@ ENDPROC(copy_user_generic_unrolled)
* Input:
* rdi destination
* rsi source
- * rdx count
+ * edx count
+ * ecx flags
*
* Output:
* eax uncopied bytes or 0 if successful.
*/
ENTRY(copy_user_generic_string)
CFI_STARTPROC
- andl %edx,%edx
- jz 4f
+ movl %ecx,%eax /* save flags */
cmpl $8,%edx
jb 2f /* less than 8 bytes, go to byte copy loop */
ALIGN_DESTINATION
@@ -250,12 +260,13 @@ ENTRY(copy_user_generic_string)
2: movl %edx,%ecx
3: rep
movsb
-4: xorl %eax,%eax
+ xorl %eax,%eax
ret

.section .fixup,"ax"
11: lea (%rdx,%rcx,8),%rcx
-12: movl %ecx,%edx /* ecx is zerorest also */
+12: movl %ecx,%edx
+ movl %eax,%ecx /* get flags back to ecx */
jmp copy_user_handle_tail
.previous

diff --git a/arch/x86/lib/copy_user_nocache_64.S b/arch/x86/lib/copy_user_nocache_64.S
index 40e0e30..8703ccd 100644
--- a/arch/x86/lib/copy_user_nocache_64.S
+++ b/arch/x86/lib/copy_user_nocache_64.S
@@ -9,11 +9,13 @@
#include <linux/linkage.h>
#include <asm/dwarf2.h>

+/* Align destination */
#define FIX_ALIGNMENT 1

#include <asm/current.h>
#include <asm/asm-offsets.h>
#include <asm/thread_info.h>
+#include <asm/uaccess_64.h>

.macro ALIGN_DESTINATION
#ifdef FIX_ALIGNMENT
@@ -24,15 +26,16 @@
subl $8,%ecx
negl %ecx
subl %ecx,%edx
-100: movb (%rsi),%al
-101: movb %al,(%rdi)
+100: movb (%rsi),%ah /* al is flags */
+101: movb %ah,(%rdi)
incq %rsi
incq %rdi
decl %ecx
jnz 100b
102:
.section .fixup,"ax"
-103: addl %r8d,%edx /* ecx is zerorest also */
+103: addl %ecx,%edx
+ movl %eax,%ecx
jmp copy_user_handle_tail
.previous

@@ -41,7 +44,7 @@
.quad 100b,103b
.quad 101b,103b
.previous
-#endif
+#endif /* FIX_ALIGNMENT */
.endm

/*
@@ -50,6 +53,7 @@
*/
ENTRY(__copy_user_nocache)
CFI_STARTPROC
+ movl %ecx,%eax /* save flags in eax */
cmpl $8,%edx
jb 20f /* less then 8 bytes, go to byte copy loop */
ALIGN_DESTINATION
@@ -87,11 +91,11 @@ ENTRY(__copy_user_nocache)
leaq 8(%rdi),%rdi
decl %ecx
jnz 18b
-20: andl %edx,%edx
+20: testl %edx,%edx
jz 23f
movl %edx,%ecx
-21: movb (%rsi),%al
-22: movb %al,(%rdi)
+21: movb (%rsi),%ah /* al is flags */
+22: movb %ah,(%rdi)
incq %rsi
incq %rdi
decl %ecx
@@ -108,7 +112,7 @@ ENTRY(__copy_user_nocache)
jmp 60f
50: movl %ecx,%edx
60: sfence
- movl %r8d,%ecx
+ movl %eax,%ecx /* get flags back to ecx*/
jmp copy_user_handle_tail
.previous

--
wbr, Vitaly

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