KVM: nested VMX: disable perf cpuid reporting

Initilization of L2 guest with -cpu host, on L1 guest with -cpu host
triggers:

(qemu) KVM: entry failed, hardware error 0x7
...
nested_vmx_run: VMCS MSR_{LOAD,STORE} unsupported

Nested VMX MSR load/store support is not sufficient to
allow perf for L2 guest.

Until properly fixed, trap CPUID and disable function 0xA.
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/cpuid.c

@@ -774,6 +774,12 @@ void kvm_cpuid(struct kvm_vcpu *vcpu, u32 *eax, u32 *ebx, u32 *ecx, u32 *edx)
 	if (!best)
 		best = check_cpuid_limit(vcpu, function, index);
 
+	/*
+	 * Perfmon not yet supported for L2 guest.
+	 */
+	if (is_guest_mode(vcpu) && function == 0xa)
+		best = NULL;
+
 	if (best) {
 		*eax = best->eax;
 		*ebx = best->ebx;

arch/x86/kvm/vmx.c

@@ -6986,6 +6986,8 @@ static bool nested_vmx_exit_handled(struct kvm_vcpu *vcpu)
 	case EXIT_REASON_TASK_SWITCH:
 		return 1;
 	case EXIT_REASON_CPUID:
+		if (kvm_register_read(vcpu, VCPU_REGS_RAX) == 0xa)
+			return 0;
 		return 1;
 	case EXIT_REASON_HLT:
 		return nested_cpu_has(vmcs12, CPU_BASED_HLT_EXITING);