[PATCH 1/2] x86: use alternatives for clear_user()

[Alexey Dobriyan <adobriyan@gmail.com>]

Alternatives allow to pick faster code: REP STOSQ, REP STOSB or else.
Default to REP STOSQ (as memset() does).

Sticking printk() into clear_user() and booting Debian showed
that it is usually executed with 4-8 bytes of alignment and
~2000-4000 bytes of length. On this scale difference should be noticeable.

Also make __clear_user() more, shall we say, "modern". Remove storing zeroes
via zeroed register, replace INC/DEC with ADD/SUB for dependency breakage.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
---

 arch/x86/lib/Makefile        |    1 
 arch/x86/lib/clear_user_64.S |   93 +++++++++++++++++++++++++++++++++++++++++++
 arch/x86/lib/usercopy_64.c   |   35 ----------------
 3 files changed, 95 insertions(+), 34 deletions(-)

--- a/arch/x86/lib/Makefile
+++ b/arch/x86/lib/Makefile
@@ -19,6 +19,7 @@ obj-$(CONFIG_SMP) += msr-smp.o cache-smp.o
 lib-y := delay.o misc.o cmdline.o
 lib-y += thunk_$(BITS).o
 lib-y += usercopy_$(BITS).o usercopy.o getuser.o putuser.o
+lib-$(CONFIG_X86_64) += clear_user_64.o
 lib-y += memcpy_$(BITS).o
 lib-$(CONFIG_RWSEM_XCHGADD_ALGORITHM) += rwsem.o
 lib-$(CONFIG_INSTRUCTION_DECODER) += insn.o inat.o
--- /dev/null
+++ b/arch/x86/lib/clear_user_64.S
@@ -0,0 +1,93 @@
+#include <linux/linkage.h>
+#include <asm/alternative-asm.h>
+#include <asm/dwarf2.h>
+#include <asm/smap.h>
+
+# unsigned long __clear_user(void __user *, unsigned long);
+ENTRY(__clear_user)
+	CFI_STARTPROC
+
+	ALTERNATIVE_2 "jmp __clear_user_movq",			\
+		"", X86_FEATURE_REP_GOOD,			\
+		"jmp __clear_user_rep_stosb", X86_FEATURE_ERMS
+
+	ASM_STAC
+	xor	%eax, %eax
+	mov	%rsi, %rcx
+	and	$7, %esi
+	shr	$3, %rcx
+1:	rep stosq
+	mov	%esi, %ecx
+2:	rep stosb
+3:
+	mov	%rcx, %rax
+	ASM_CLAC
+	ret
+
+	.section .fixup,"ax"
+4:	lea	(%rsi,%rcx,8),%rcx
+	jmp	3b
+	.previous
+
+	_ASM_EXTABLE(1b,4b)
+	_ASM_EXTABLE(2b,3b)
+
+	CFI_ENDPROC
+ENDPROC(__clear_user)
+
+ENTRY(__clear_user_movq)
+	CFI_STARTPROC
+
+	ASM_STAC
+	mov	%rsi, %rcx
+	and	$7, %esi
+	shr	$3, %rcx
+	jz	2f
+	.p2align 4
+1:
+	movq	$0, (%rdi)
+	add	$8, %rdi
+	sub	$1, %rcx
+	jnz	1b
+2:
+	mov	%esi, %ecx
+	test	%ecx, %ecx
+	jz	4f
+	.p2align 4
+3:
+	movb	$0, (%rdi)
+	add	$1, %rdi
+	sub	$1, %ecx
+	jnz	3b
+4:
+	mov	%rcx, %rax
+	ASM_CLAC
+	ret
+
+	.section .fixup,"ax"
+5:	lea	(%rsi,%rcx,8),%rcx
+	jmp	4b
+	.previous
+
+	_ASM_EXTABLE(1b,5b)
+	_ASM_EXTABLE(3b,4b)
+
+	CFI_ENDPROC
+ENDPROC(__clear_user_movq)
+
+ENTRY(__clear_user_rep_stosb)
+	CFI_STARTPROC
+
+	ASM_STAC
+	xor	%eax, %eax
+	mov	%rsi, %rcx
+1:	rep stosb
+2:
+	mov	%rcx, %rax
+	ASM_CLAC
+	ret
+
+	_ASM_EXTABLE(1b,2b)
+
+	CFI_ENDPROC
+ENDPROC(__clear_user_rep_stosb)
--- a/arch/x86/lib/usercopy_64.c
+++ b/arch/x86/lib/usercopy_64.c
@@ -12,40 +12,6 @@
  * Zero Userspace
  */
 
-unsigned long __clear_user(void __user *addr, unsigned long size)
-{
-	long __d0;
-	might_fault();
-	/* no memory constraint because it doesn't change any memory gcc knows
-	   about */
-	stac();
-	asm volatile(
-		"	testq  %[size8],%[size8]\n"
-		"	jz     4f\n"
-		"0:	movq %[zero],(%[dst])\n"
-		"	addq   %[eight],%[dst]\n"
-		"	decl %%ecx ; jnz   0b\n"
-		"4:	movq  %[size1],%%rcx\n"
-		"	testl %%ecx,%%ecx\n"
-		"	jz     2f\n"
-		"1:	movb   %b[zero],(%[dst])\n"
-		"	incq   %[dst]\n"
-		"	decl %%ecx ; jnz  1b\n"
-		"2:\n"
-		".section .fixup,\"ax\"\n"
-		"3:	lea 0(%[size1],%[size8],8),%[size8]\n"
-		"	jmp 2b\n"
-		".previous\n"
-		_ASM_EXTABLE(0b,3b)
-		_ASM_EXTABLE(1b,2b)
-		: [size8] "=&c"(size), [dst] "=&D" (__d0)
-		: [size1] "r"(size & 7), "[size8]" (size / 8), "[dst]"(addr),
-		  [zero] "r" (0UL), [eight] "r" (8UL));
-	clac();
-	return size;
-}
-EXPORT_SYMBOL(__clear_user);
-
 unsigned long clear_user(void __user *to, unsigned long n)
 {
 	if (access_ok(VERIFY_WRITE, to, n))
@@ -53,6 +19,7 @@ unsigned long clear_user(void __user *to, unsigned long n)
 	return n;
 }
 EXPORT_SYMBOL(clear_user);
+EXPORT_SYMBOL(__clear_user);
 
 unsigned long copy_in_user(void __user *to, const void __user *from, unsigned len)
 {
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