linux-kernel - [PATCH] x86: branchless clear

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Date:   Mon, 22 Aug 2016 23:52:29 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     x86@...nel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] x86: branchless clear_page()

Apply alternatives at the call site instead of function body.
Save branch per clean page.

Bonus: tell gcc to not flush whole shebang of registers,
just RDI, RAX, RCX.

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

 arch/x86/include/asm/page_64.h |   16 +++++++++++++++-
 arch/x86/lib/clear_page_64.S   |   18 ++++++------------
 2 files changed, 21 insertions(+), 13 deletions(-)

--- a/arch/x86/include/asm/page_64.h
+++ b/arch/x86/include/asm/page_64.h
@@ -4,6 +4,7 @@
 #include <asm/page_64_types.h>
 
 #ifndef __ASSEMBLY__
+#include <asm/alternative.h>
 
 /* duplicated to the one in bootmem.h */
 extern unsigned long max_pfn;
@@ -34,7 +35,20 @@ extern unsigned long __phys_addr_symbol(unsigned long);
 #define pfn_valid(pfn)          ((pfn) < max_pfn)
 #endif
 
-void clear_page(void *page);
+void clear_page_mov(void *page);
+void clear_page_rep_stosq(void *page);
+void clear_page_rep_stosb(void *page);
+static __always_inline void clear_page(void *page)
+{
+	alternative_call_2(
+		clear_page_mov,
+		clear_page_rep_stosq, X86_FEATURE_REP_GOOD,
+		clear_page_rep_stosb, X86_FEATURE_ERMS,
+		"=D" (page),
+		"0" (page)
+		: "rax", "rcx", "memory"
+	);
+}
 void copy_page(void *to, void *from);
 
 #endif	/* !__ASSEMBLY__ */
--- a/arch/x86/lib/clear_page_64.S
+++ b/arch/x86/lib/clear_page_64.S
@@ -1,6 +1,4 @@
 #include <linux/linkage.h>
-#include <asm/cpufeatures.h>
-#include <asm/alternative-asm.h>
 
 /*
  * Most CPUs support enhanced REP MOVSB/STOSB instructions. It is
@@ -13,18 +11,14 @@
  * Zero a page.
  * %rdi	- page
  */
-ENTRY(clear_page)
-
-	ALTERNATIVE_2 "jmp clear_page_orig", "", X86_FEATURE_REP_GOOD, \
-		      "jmp clear_page_c_e", X86_FEATURE_ERMS
-
+ENTRY(clear_page_rep_stosq)
 	movl $4096/8,%ecx
 	xorl %eax,%eax
 	rep stosq
 	ret
-ENDPROC(clear_page)
+ENDPROC(clear_page_rep_stosq)
 
-ENTRY(clear_page_orig)
+ENTRY(clear_page_mov)
 
 	xorl   %eax,%eax
 	movl   $4096/64,%ecx
@@ -44,11 +38,11 @@ ENTRY(clear_page_orig)
 	jnz	.Lloop
 	nop
 	ret
-ENDPROC(clear_page_orig)
+ENDPROC(clear_page_mov)
 
-ENTRY(clear_page_c_e)
+ENTRY(clear_page_rep_stosb)
 	movl $4096,%ecx
 	xorl %eax,%eax
 	rep stosb
 	ret
-ENDPROC(clear_page_c_e)
+ENDPROC(clear_page_rep_stosb)