linux-kernel - [PATCH v2 13/15] x86/lib/memset_64.S: Convert to ALTERNATIVE

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Date:	Tue, 24 Feb 2015 12:14:55 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	X86 ML <x86@...nel.org>
Cc:	Andy Lutomirski <luto@...capital.net>,
	LKML <linux-kernel@...r.kernel.org>
Subject: [PATCH v2 13/15] x86/lib/memset_64.S: Convert to ALTERNATIVE_2 macro

From: Borislav Petkov <bp@...e.de>

Make alternatives replace single JMPs instead of whole memset functions,
thus decreasing the amount of instructions copied during patching time
at boot.

While at it, make it use the REP_GOOD version by default which means
alternatives NOP out the JMP to the other versions, as REP_GOOD is set
by default on the majority of relevant x86 processors.

Signed-off-by: Borislav Petkov <bp@...e.de>
---
 arch/x86/lib/memset_64.S | 61 +++++++++++++++++++-----------------------------
 1 file changed, 24 insertions(+), 37 deletions(-)

diff --git a/arch/x86/lib/memset_64.S b/arch/x86/lib/memset_64.S
index f6153c1cdddc..93118fb23976 100644
--- a/arch/x86/lib/memset_64.S
+++ b/arch/x86/lib/memset_64.S
@@ -5,19 +5,30 @@
 #include <asm/cpufeature.h>
 #include <asm/alternative-asm.h>
 
+.weak memset
+
 /*
  * ISO C memset - set a memory block to a byte value. This function uses fast
  * string to get better performance than the original function. The code is
  * simpler and shorter than the orignal function as well.
- *	
+ *
  * rdi   destination
- * rsi   value (char) 
- * rdx   count (bytes) 
- * 
+ * rsi   value (char)
+ * rdx   count (bytes)
+ *
  * rax   original destination
- */	
-	.section .altinstr_replacement, "ax", @progbits
-.Lmemset_c:
+ */
+ENTRY(memset)
+ENTRY(__memset)
+	/*
+	 * Some CPUs support enhanced REP MOVSB/STOSB feature. It is recommended
+	 * to use it when possible. If not available, use fast string instructions.
+	 *
+	 * Otherwise, use original memset function.
+	 */
+	ALTERNATIVE_2 "jmp memset_orig", "", X86_FEATURE_REP_GOOD, \
+		      "jmp memset_erms", X86_FEATURE_ERMS
+
 	movq %rdi,%r9
 	movq %rdx,%rcx
 	andl $7,%edx
@@ -31,8 +42,8 @@
 	rep stosb
 	movq %r9,%rax
 	ret
-.Lmemset_e:
-	.previous
+ENDPROC(memset)
+ENDPROC(__memset)
 
 /*
  * ISO C memset - set a memory block to a byte value. This function uses
@@ -45,21 +56,16 @@
  *
  * rax   original destination
  */
-	.section .altinstr_replacement, "ax", @progbits
-.Lmemset_c_e:
+ENTRY(memset_erms)
 	movq %rdi,%r9
 	movb %sil,%al
 	movq %rdx,%rcx
 	rep stosb
 	movq %r9,%rax
 	ret
-.Lmemset_e_e:
-	.previous
-
-.weak memset
+ENDPROC(memset_erms)
 
-ENTRY(memset)
-ENTRY(__memset)
+ENTRY(memset_orig)
 	CFI_STARTPROC
 	movq %rdi,%r10
 
@@ -134,23 +140,4 @@ ENTRY(__memset)
 	jmp .Lafter_bad_alignment
 .Lfinal:
 	CFI_ENDPROC
-ENDPROC(memset)
-ENDPROC(__memset)
-
-	/* Some CPUs support enhanced REP MOVSB/STOSB feature.
-	 * It is recommended to use this when possible.
-	 *
-	 * If enhanced REP MOVSB/STOSB feature is not available, use fast string
-	 * instructions.
-	 *
-	 * Otherwise, use original memset function.
-	 *
-	 * In .altinstructions section, ERMS feature is placed after REG_GOOD
-         * feature to implement the right patch order.
-	 */
-	.section .altinstructions,"a"
-	altinstruction_entry __memset,.Lmemset_c,X86_FEATURE_REP_GOOD,\
-			     .Lfinal-__memset,.Lmemset_e-.Lmemset_c,0
-	altinstruction_entry __memset,.Lmemset_c_e,X86_FEATURE_ERMS, \
-			     .Lfinal-__memset,.Lmemset_e_e-.Lmemset_c_e,0
-	.previous
+ENDPROC(memset_orig)
-- 
2.2.0.33.gc18b867

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