KVM: x86: skip host CPUID call for hypervisor leaves

Hypervisor leaves are always synthesized by __do_cpuid_func; just return
zeroes and do not ask the host.  Even on nested virtualization, a value
from another hypervisor would be bogus, because all hypercalls and MSRs
are processed by KVM.
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/cpuid.c

@@ -712,9 +712,17 @@ static struct kvm_cpuid_entry2 *do_host_cpuid(struct kvm_cpuid_array *array,
 
 	entry = &array->entries[array->nent++];
 
+	memset(entry, 0, sizeof(*entry));
 	entry->function = function;
 	entry->index = index;
-	entry->flags = 0;
+	switch (function & 0xC0000000) {
+	case 0x40000000:
+		/* Hypervisor leaves are always synthesized by __do_cpuid_func.  */
+		return entry;
+
+	default:
+		break;
+	}
 
 	cpuid_count(entry->function, entry->index,
 		    &entry->eax, &entry->ebx, &entry->ecx, &entry->edx);