x86: PAT Update validate_pat_support for intel CPUs

Pentium III and Core Solo/Duo CPUs have an erratum
" Page with PAT set to WC while associated MTRR is UC may consolidate to UC "
which can result in WC setting in PAT to be ineffective. We will disable
PAT on such CPUs, so that we can continue to use MTRR WC setting.

Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 15 insertions, 2 deletions

diff --git a/arch/x86/kernel/cpu/addon_cpuid_features.c b/arch/x86/kernel/cpu/addon_cpuid_features.c
index 84a8220a6072..a6ef672adbba 100644
--- a/arch/x86/kernel/cpu/addon_cpuid_features.c
+++ b/arch/x86/kernel/cpu/addon_cpuid_features.c
@@ -56,9 +56,22 @@ void __cpuinit validate_pat_support(struct cpuinfo_x86 *c)
 
         switch (c->x86_vendor) {
         case X86_VENDOR_INTEL:
-                if (c->x86 == 0xF || (c->x86 == 6 && c->x86_model >= 15))
+                /*
+                 * There is a known erratum on Pentium III and Core Solo
+                 * and Core Duo CPUs.
+                 * " Page with PAT set to WC while associated MTRR is UC
+                 *   may consolidate to UC "
+                 * Because of this erratum, it is better to stick with
+                 * setting WC in MTRR rather than using PAT on these CPUs.
+                 *
+                 * Enable PAT WC only on P4, Core 2 or later CPUs.
+                 */
+                if (c->x86 > 0x6 || (c->x86 == 6 && c->x86_model >= 15))
                         return;
-                break;
+
+                pat_disable("PAT WC disabled due to known CPU erratum.");
+                return;
+
         case X86_VENDOR_AMD:
         case X86_VENDOR_CENTAUR:
         case X86_VENDOR_TRANSMETA: