From: Borislav Petkov <b...@suse.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 <b...@suse.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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