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 <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/kernel/cpu/addon_cpuid_features.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: