x86, cpu, amd: Fix WC+ workaround for older virtual hosts

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 <[email protected]>
Cc: Boris Ostrovsky <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: H. Peter Anvin <[email protected]>

1 files 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);