[PATCH 1/2] x86, CPU, AMD: Fix WC+ workaround for older hosts

[Borislav Petkov]

From: Borislav Petkov <bp@xxxxxxx>

The WC+ workaround for F10h introduces a new MSR and kvm host #GPs
on accesses to unknown MSRs if paravirt is not compiled in. Use the
exception-handling MSR accessors so as not to break 3.8 and later guests
booting on older hosts.

Remove a redundant family check while at it.

Cc: Gleb Natapov <gleb@xxxxxxxxxx>
Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Link: http://lkml.kernel.org/r/20130219153655.GD26748@xxxxxxx
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
---
 arch/x86/kernel/cpu/amd.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index 721ef3208eb5..163af4a91d09 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -723,12 +723,14 @@ static void __cpuinit init_amd(struct cpuinfo_x86 *c)
 		 * performance degradation for certain nested-paging guests.
 		 * Prevent this conversion by clearing bit 24 in
 		 * MSR_AMD64_BU_CFG2.
+		 *
+		 * NOTE: we want to use the _safe accessors so as not to #GP kvm
+		 * guests on older kvm hosts.
 		 */
-		if (c->x86 == 0x10) {
-			rdmsrl(MSR_AMD64_BU_CFG2, value);
-			value &= ~(1ULL << 24);
-			wrmsrl(MSR_AMD64_BU_CFG2, value);
-		}
+
+		rdmsrl_safe(MSR_AMD64_BU_CFG2, &value);
+		value &= ~(1ULL << 24);
+		wrmsrl_safe(MSR_AMD64_BU_CFG2, value);
 	}
 
 	rdmsr_safe(MSR_AMD64_PATCH_LEVEL, &c->microcode, &dummy);
-- 
1.8.1.3.535.ga923c31

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