x86, AMD: Enable WC+ memory type on family 10 processors

In some cases BIOS may not enable WC+ memory type on family 10
processors, instead converting what would be WC+ memory to CD type.
On guests using nested pages this could result in performance
degradation. This patch enables WC+.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@amd.com>
Link: http://lkml.kernel.org/r/1359495169-23278-1-git-send-email-ostr@amd64.org
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

arch/x86/include/uapi/asm/msr-index.h

@@ -173,6 +173,7 @@
 #define MSR_AMD64_OSVW_ID_LENGTH	0xc0010140
 #define MSR_AMD64_OSVW_STATUS		0xc0010141
 #define MSR_AMD64_DC_CFG		0xc0011022
+#define MSR_AMD64_BU_CFG2		0xc001102a
 #define MSR_AMD64_IBSFETCHCTL		0xc0011030
 #define MSR_AMD64_IBSFETCHLINAD		0xc0011031
 #define MSR_AMD64_IBSFETCHPHYSAD	0xc0011032

arch/x86/kernel/cpu/amd.c

@@ -698,13 +698,11 @@ static void __cpuinit init_amd(struct cpuinfo_x86 *c)
 	if (c->x86 > 0x11)
 		set_cpu_cap(c, X86_FEATURE_ARAT);
 
-	/*
-	 * Disable GART TLB Walk Errors on Fam10h. We do this here
-	 * because this is always needed when GART is enabled, even in a
-	 * kernel which has no MCE support built in.
-	 */
 	if (c->x86 == 0x10) {
 		/*
+		 * Disable GART TLB Walk Errors on Fam10h. We do this here
+		 * because this is always needed when GART is enabled, even in a
+		 * kernel which has no MCE support built in.
 		 * BIOS should disable GartTlbWlk Errors themself. If
 		 * it doesn't do it here as suggested by the BKDG.
 		 *
@@ -718,6 +716,19 @@ static void __cpuinit init_amd(struct cpuinfo_x86 *c)
 			mask |= (1 << 10);
 			wrmsrl_safe(MSR_AMD64_MCx_MASK(4), mask);
 		}
+
+		/*
+		 * On family 10h BIOS may not have properly enabled WC+ support,
+		 * causing it to be converted to CD memtype. This may result in
+		 * performance degradation for certain nested-paging guests.
+		 * Prevent this conversion by clearing bit 24 in
+		 * MSR_AMD64_BU_CFG2.
+		 */
+		if (c->x86 == 0x10) {
+			rdmsrl(MSR_AMD64_BU_CFG2, value);
+			value &= ~(1ULL << 24);
+			wrmsrl(MSR_AMD64_BU_CFG2, value);
+		}
 	}
 
 	rdmsr_safe(MSR_AMD64_PATCH_LEVEL, &c->microcode, &dummy);