linux-kernel - [PATCH 5/8] x86: Make old K8 swapgs workaround conditional

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Date:	Tue, 14 Oct 2014 22:11:48 -0700
From:	Andi Kleen <andi@...stfloor.org>
To:	linux-kernel@...r.kernel.org
Cc:	x86@...nel.org, Andi Kleen <ak@...ux.intel.com>
Subject: [PATCH 5/8] x86: Make old K8 swapgs workaround conditional

From: Andi Kleen <ak@...ux.intel.com>

Every gs selector/index reload always paid an extra MFENCE
between the two SWAPGS. This was to work around an old
bug in early K8 steppings.  All other CPUs don't need the extra
mfence. Patch the extra MFENCE only in for K8.

Signed-off-by: Andi Kleen <ak@...ux.intel.com>
---
 arch/x86/include/asm/cpufeature.h |  1 +
 arch/x86/kernel/cpu/amd.c         |  3 +++
 arch/x86/kernel/entry_64.S        | 10 +++++++++-
 3 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
index bb9b258..5125c95 100644
--- a/arch/x86/include/asm/cpufeature.h
+++ b/arch/x86/include/asm/cpufeature.h
@@ -102,6 +102,7 @@
 #define X86_FEATURE_APERFMPERF	( 3*32+28) /* APERFMPERF */
 #define X86_FEATURE_EAGER_FPU	( 3*32+29) /* "eagerfpu" Non lazy FPU restore */
 #define X86_FEATURE_NONSTOP_TSC_S3 ( 3*32+30) /* TSC doesn't stop in S3 state */
+#define X86_FEATURE_SWAPGS_MFENCE (3*32+31) /* SWAPGS may need MFENCE */
 
 /* Intel-defined CPU features, CPUID level 0x00000001 (ecx), word 4 */
 #define X86_FEATURE_XMM3	( 4*32+ 0) /* "pni" SSE-3 */
diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index 60e5497..85988e8 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -544,6 +544,9 @@ static void init_amd_k8(struct cpuinfo_x86 *c)
 	if ((level >= 0x0f48 && level < 0x0f50) || level >= 0x0f58)
 		set_cpu_cap(c, X86_FEATURE_REP_GOOD);
 
+	/* Early steppings needed a mfence on swapgs. */
+	set_cpu_cap(c, X86_FEATURE_SWAPGS_MFENCE);
+
 	/*
 	 * Some BIOSes incorrectly force this feature, but only K8 revision D
 	 * (model = 0x14) and later actually support it.
diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 43b46a1..5913360 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -1216,13 +1216,21 @@ ENTRY(native_load_gs_index)
 	SWAPGS
 gs_change:
 	movl %edi,%gs
-2:	mfence		/* workaround */
+2:	ASM_NOP3	/* may be replaced with mfence */
 	SWAPGS
 	popfq_cfi
 	ret
 	CFI_ENDPROC
 END(native_load_gs_index)
 
+	/* Early K8 systems needed an mfence after swapgs to workaround a bug */
+	.section .altinstr_replacement,"ax"
+3:	mfence
+	.previous
+	.section .altinstructions,"a"
+	altinstruction_entry 2b,3b,X86_FEATURE_SWAPGS_MFENCE,3,3
+	.previous
+
 	_ASM_EXTABLE(gs_change,bad_gs)
 	.section .fixup,"ax"
 	/* running with kernelgs */
-- 
1.9.3

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