KVM: x86: add support for CPUID leaf 0x80000021

CPUID leaf 0x80000021 defines some features (or lack of bugs) of AMD
processors.  Expose the ones that make sense via KVM_GET_SUPPORTED_CPUID.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/cpuid.c

@@ -1068,7 +1068,7 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
 		entry->edx = 0;
 		break;
 	case 0x80000000:
-		entry->eax = min(entry->eax, 0x8000001f);
+		entry->eax = min(entry->eax, 0x80000021);
 		break;
 	case 0x80000001:
 		cpuid_entry_override(entry, CPUID_8000_0001_EDX);
@@ -1139,6 +1139,23 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
 			entry->ebx &= ~GENMASK(11, 6);
 		}
 		break;
+	case 0x80000020:
+		entry->eax = entry->ebx = entry->ecx = entry->edx = 0;
+		break;
+	case 0x80000021:
+		entry->ebx = entry->ecx = entry->edx = 0;
+		/*
+		 * Pass down these bits:
+		 *    EAX      0      NNDBP, Processor ignores nested data breakpoints
+		 *    EAX      2      LAS, LFENCE always serializing
+		 *    EAX      6      NSCB, Null selector clear base
+		 *
+		 * Other defined bits are for MSRs that KVM does not expose:
+		 *   EAX      3      SPCL, SMM page configuration lock
+		 *   EAX      13     PCMSR, Prefetch control MSR
+		 */
+		entry->eax &= BIT(0) | BIT(2) | BIT(6);
+		break;
 	/*Add support for Centaur's CPUID instruction*/
 	case 0xC0000000:
 		/*Just support up to 0xC0000004 now*/