linux-kernel - [PATCH v2] mce: use safe MSR accesses

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Date:	Fri, 13 Mar 2015 11:03:39 -0500
From:	<jesse.larrew@....com>
To:	<x86@...nel.org>
CC:	Joel Schopp <joel.schopp@....com>, Tony Luck <tony.luck@...el.com>,
	Borislav Petkov <bp@...en8.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Ingo Molnar" <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>,
	<linux-edac@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	Jesse Larrew <jesse.larrew@....com>
Subject: [PATCH v2] mce: use safe MSR accesses

From: Jesse Larrew <jesse.larrew@....com>

Certain MSRs are only relevant to a kernel in host mode, and kvm had
chosen not to implement these MSRs at all for guests. If a guest kernel
ever tried to access these MSRs, the result was a general protection
fault.

KVM will be separately patched to return 0 when these MSRs are read,
and this patch ensures that MSR accesses are tolerant of exceptions.

Signed-off-by: Jesse Larrew <jesse.larrew@....com>
---
 arch/x86/kernel/cpu/mcheck/mce.c | 11 +++--------
 1 file changed, 3 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/cpu/mcheck/mce.c b/arch/x86/kernel/cpu/mcheck/mce.c
index 61a9668ce..2737ced 100644
--- a/arch/x86/kernel/cpu/mcheck/mce.c
+++ b/arch/x86/kernel/cpu/mcheck/mce.c
@@ -1540,7 +1540,7 @@ static int __mcheck_cpu_apply_quirks(struct cpuinfo_x86 *c)
 		 if (c->x86 == 0x15 &&
 		     (c->x86_model >= 0x10 && c->x86_model <= 0x1f)) {
 			 int i;
-			 u64 val, hwcr;
+			 u64 hwcr;
 			 bool need_toggle;
 			 u32 msrs[] = {
 				0x00000413, /* MC4_MISC0 */
@@ -1556,13 +1556,8 @@ static int __mcheck_cpu_apply_quirks(struct cpuinfo_x86 *c)
 				 wrmsrl(MSR_K7_HWCR, hwcr | BIT(18));
 
 			 for (i = 0; i < ARRAY_SIZE(msrs); i++) {
-				 rdmsrl(msrs[i], val);
-
-				 /* CntP bit set? */
-				 if (val & BIT_64(62)) {
-					val &= ~BIT_64(62);
-					wrmsrl(msrs[i], val);
-				 }
+				 /* Clear CntP bit safely */
+				 msr_clear_bit(msrs[i], 62);
 			 }
 
 			 /* restore old settings */
-- 
1.9.1

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