KVM: Fix MAXPHYADDR calculation when cpuid does not support it

MAXPHYADDR is derived from cpuid 0x80000008, but when that isn't present, we
get some random value.

Fix by checking first that cpuid 0x80000008 is supported.
Acked-by: NPekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: NAvi Kivity <avi@redhat.com>
Signed-off-by: NMarcelo Tosatti <mtosatti@redhat.com>

arch/x86/kvm/x86.c

@@ -4171,9 +4171,13 @@ int cpuid_maxphyaddr(struct kvm_vcpu *vcpu)
 {
 	struct kvm_cpuid_entry2 *best;
 
+	best = kvm_find_cpuid_entry(vcpu, 0x80000000, 0);
+	if (!best || best->eax < 0x80000008)
+		goto not_found;
 	best = kvm_find_cpuid_entry(vcpu, 0x80000008, 0);
 	if (best)
 		return best->eax & 0xff;
+not_found:
 	return 36;
 }