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 <gleb@redhat.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: http://lkml.kernel.org/r/1361298793-31834-1-git-send-email-bp@alien8.de
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

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);