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Subject[PATCH] x86-64: modify memcpy()/memset() alternatives mechanism
In order to avoid unnecessary chains of branches, rather than
implementing memcpy()/memset()'s access to their alternative
implementations via a jump, patch the (larger) original function
directly.

The memcpy() part of this is slightly subtle: While alternative
instruction patching does itself use memcpy(), with the replacement
block being less than 64-bytes in size the main loop of the original
function doesn't get used for copying memcpy_c() over memcpy(), and
hence we can safely write over its beginning.

Also note that the CFI annotations are fine for both variants of each
of the functions.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Cc: Nick Piggin <npiggin@suse.de>

---
arch/x86/lib/memcpy_64.S | 23 ++++++++---------------
arch/x86/lib/memset_64.S | 18 ++++++------------
2 files changed, 14 insertions(+), 27 deletions(-)

--- linux-2.6.33-rc1/arch/x86/lib/memcpy_64.S 2009-06-10 05:05:27.000000000 +0200
+++ 2.6.33-rc1-x86_64-memcpy-memset-alternative/arch/x86/lib/memcpy_64.S 2009-11-16 16:10:22.000000000 +0100
@@ -20,12 +20,11 @@
/*
* memcpy_c() - fast string ops (REP MOVSQ) based variant.
*
- * Calls to this get patched into the kernel image via the
+ * This gets patched over the unrolled variant (below) via the
* alternative instructions framework:
*/
- ALIGN
-memcpy_c:
- CFI_STARTPROC
+ .section .altinstr_replacement, "ax", @progbits
+.Lmemcpy_c:
movq %rdi, %rax

movl %edx, %ecx
@@ -35,8 +34,8 @@ memcpy_c:
movl %edx, %ecx
rep movsb
ret
- CFI_ENDPROC
-ENDPROC(memcpy_c)
+.Lmemcpy_e:
+ .previous

ENTRY(__memcpy)
ENTRY(memcpy)
@@ -128,16 +127,10 @@ ENDPROC(__memcpy)
* It is also a lot simpler. Use this when possible:
*/

- .section .altinstr_replacement, "ax"
-1: .byte 0xeb /* jmp <disp8> */
- .byte (memcpy_c - memcpy) - (2f - 1b) /* offset */
-2:
- .previous
-
.section .altinstructions, "a"
.align 8
.quad memcpy
- .quad 1b
+ .quad .Lmemcpy_c
.byte X86_FEATURE_REP_GOOD

/*
@@ -145,6 +138,6 @@ ENDPROC(__memcpy)
* so it is silly to overwrite itself with nops - reboot is the
* only outcome...
*/
- .byte 2b - 1b
- .byte 2b - 1b
+ .byte .Lmemcpy_e - .Lmemcpy_c
+ .byte .Lmemcpy_e - .Lmemcpy_c
.previous
--- linux-2.6.33-rc1/arch/x86/lib/memset_64.S 2008-01-24 23:58:37.000000000 +0100
+++ 2.6.33-rc1-x86_64-memcpy-memset-alternative/arch/x86/lib/memset_64.S 2009-11-16 16:03:03.000000000 +0100
@@ -12,9 +12,8 @@
*
* rax original destination
*/
- ALIGN
-memset_c:
- CFI_STARTPROC
+ .section .altinstr_replacement, "ax", @progbits
+.Lmemset_c:
movq %rdi,%r9
movl %edx,%r8d
andl $7,%r8d
@@ -29,8 +28,8 @@ memset_c:
rep stosb
movq %r9,%rax
ret
- CFI_ENDPROC
-ENDPROC(memset_c)
+.Lmemset_e:
+ .previous

ENTRY(memset)
ENTRY(__memset)
@@ -118,16 +117,11 @@ ENDPROC(__memset)

#include <asm/cpufeature.h>

- .section .altinstr_replacement,"ax"
-1: .byte 0xeb /* jmp <disp8> */
- .byte (memset_c - memset) - (2f - 1b) /* offset */
-2:
- .previous
.section .altinstructions,"a"
.align 8
.quad memset
- .quad 1b
+ .quad .Lmemset_c
.byte X86_FEATURE_REP_GOOD
.byte .Lfinal - memset
- .byte 2b - 1b
+ .byte .Lmemset_e - .Lmemset_c
.previous




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