linux-kernel - [PATCH v1 10/12] x86, alternatives: Convert clear_page

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Message-Id: <1422987390-17878-11-git-send-email-bp@alien8.de>
Date:	Tue,  3 Feb 2015 19:16:28 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	X86 ML <x86@...nel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: [PATCH v1 10/12] x86, alternatives: Convert clear_page_64.S

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

Move clear_page() up so that we can get 2-byte forward JMPs when
patching:

[    2.388007] apply_alternatives: feat: 3*32+16, old: (ffffffff8130adb0, len: 5), repl: (ffffffff81d0b859, len: 5)
[    2.390992] ffffffff8130adb0: alt_insn: 90 90 90 90 90
[    2.393169] recompute_jump: new_displ: 0x0000003e
[    2.394628] ffffffff81d0b859: rpl_insn: eb 3e 66 66 90

even though compiler generated a 5-byte JMPs which we padded with 5
NOPs:

old insn VA: 0xffffffff8130adb0, CPU feat: X86_FEATURE_REP_GOOD, size: 5
clear_page:

ffffffff8130adb0 <clear_page>:
ffffffff8130adb0:       90                      nop
ffffffff8130adb1:       90                      nop
ffffffff8130adb2:       90                      nop
ffffffff8130adb3:       90                      nop
ffffffff8130adb4:       90                      nop
repl insn: 0xffffffff81d0b859, size: 5
 ffffffff81d0b859:      e9 92 f5 5f ff          jmpq ffffffff8130adf0

ffffffff8130adb0 <clear_page>:
ffffffff8130adb0:       e9 92 f5 5f ff          jmpq ffffffff8090a347

old insn VA: 0xffffffff8130adb0, CPU feat: X86_FEATURE_ERMS, size: 5
clear_page:

ffffffff8130adb0 <clear_page>:
ffffffff8130adb0:       90                      nop
ffffffff8130adb1:       90                      nop
ffffffff8130adb2:       90                      nop
ffffffff8130adb3:       90                      nop
ffffffff8130adb4:       90                      nop
repl insn: 0xffffffff81d0b85e, size: 5
 ffffffff81d0b85e:      e9 9d f5 5f ff          jmpq ffffffff8130ae00

ffffffff8130adb0 <clear_page>:
ffffffff8130adb0:       e9 9d f5 5f ff          jmpq ffffffff8090a352

Also, make the REP_GOOD version be the default as the majority of
machines set REP_GOOD. This way we get to save ourselves the JMP.

Signed-off-by: Borislav Petkov <bp@...e.de>
---
 arch/x86/lib/clear_page_64.S | 66 ++++++++++++++++++--------------------------
 1 file changed, 27 insertions(+), 39 deletions(-)

diff --git a/arch/x86/lib/clear_page_64.S b/arch/x86/lib/clear_page_64.S
index f2145cfa12a6..e67e579c93bd 100644
--- a/arch/x86/lib/clear_page_64.S
+++ b/arch/x86/lib/clear_page_64.S
@@ -1,31 +1,35 @@
 #include <linux/linkage.h>
 #include <asm/dwarf2.h>
+#include <asm/cpufeature.h>
 #include <asm/alternative-asm.h>
 
 /*
- * Zero a page. 	
- * rdi	page
- */			
-ENTRY(clear_page_c)
+ * Most CPUs support enhanced REP MOVSB/STOSB instructions. It is
+ * recommended to use this when possible and we do use them by default.
+ * If enhanced REP MOVSB/STOSB is not available, try to use fast string.
+ * Otherwise, use original.
+ */
+
+/*
+ * Zero a page.
+ * %rdi	- page
+ */
+ENTRY(clear_page)
 	CFI_STARTPROC
+
+	ALTERNATIVE_2 "jmp clear_page_orig", "", X86_FEATURE_REP_GOOD, \
+		      "jmp clear_page_c_e", X86_FEATURE_ERMS
+
 	movl $4096/8,%ecx
 	xorl %eax,%eax
 	rep stosq
 	ret
 	CFI_ENDPROC
-ENDPROC(clear_page_c)
+ENDPROC(clear_page)
 
-ENTRY(clear_page_c_e)
+ENTRY(clear_page_orig)
 	CFI_STARTPROC
-	movl $4096,%ecx
-	xorl %eax,%eax
-	rep stosb
-	ret
-	CFI_ENDPROC
-ENDPROC(clear_page_c_e)
 
-ENTRY(clear_page)
-	CFI_STARTPROC
 	xorl   %eax,%eax
 	movl   $4096/64,%ecx
 	.p2align 4
@@ -45,29 +49,13 @@ ENTRY(clear_page)
 	nop
 	ret
 	CFI_ENDPROC
-.Lclear_page_end:
-ENDPROC(clear_page)
-
-	/*
-	 * Some CPUs support enhanced REP MOVSB/STOSB instructions.
-	 * It is recommended to use this when possible.
-	 * If enhanced REP MOVSB/STOSB is not available, try to use fast string.
-	 * Otherwise, use original function.
-	 *
-	 */
+ENDPROC(clear_page_orig)
 
-#include <asm/cpufeature.h>
-
-	.section .altinstr_replacement,"ax"
-1:	.byte 0xeb					/* jmp <disp8> */
-	.byte (clear_page_c - clear_page) - (2f - 1b)	/* offset */
-2:	.byte 0xeb					/* jmp <disp8> */
-	.byte (clear_page_c_e - clear_page) - (3f - 2b)	/* offset */
-3:
-	.previous
-	.section .altinstructions,"a"
-	altinstruction_entry clear_page,1b,X86_FEATURE_REP_GOOD,\
-			     .Lclear_page_end-clear_page, 2b-1b
-	altinstruction_entry clear_page,2b,X86_FEATURE_ERMS,   \
-			     .Lclear_page_end-clear_page,3b-2b
-	.previous
+ENTRY(clear_page_c_e)
+	CFI_STARTPROC
+	movl $4096,%ecx
+	xorl %eax,%eax
+	rep stosb
+	ret
+	CFI_ENDPROC
+ENDPROC(clear_page_c_e)
-- 
2.2.0.33.gc18b867

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