KVM: x86: hyper-v: Prepare to check access to Hyper-V hypercalls

Introduce hv_check_hypercallr_access() to check if the particular hypercall
should be available to guest, this will be used with
KVM_CAP_HYPERV_ENFORCE_CPUID mode.

No functional change intended.
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210521095204.2161214-21-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/hyperv.c

@@ -2090,6 +2090,11 @@ static void kvm_hv_hypercall_read_xmm(struct kvm_hv_hcall *hc)
 	kvm_fpu_put();
 }
 
+static bool hv_check_hypercall_access(struct kvm_vcpu_hv *hv_vcpu, u16 code)
+{
+	return true;
+}
+
 int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
 {
 	struct kvm_hv_hcall hc;
@@ -2132,6 +2137,11 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
 	trace_kvm_hv_hypercall(hc.code, hc.fast, hc.rep_cnt, hc.rep_idx,
 			       hc.ingpa, hc.outgpa);
 
+	if (unlikely(!hv_check_hypercall_access(to_hv_vcpu(vcpu), hc.code))) {
+		ret = HV_STATUS_ACCESS_DENIED;
+		goto hypercall_complete;
+	}
+
 	switch (hc.code) {
 	case HVCALL_NOTIFY_LONG_SPIN_WAIT:
 		if (unlikely(hc.rep)) {
@@ -2238,6 +2248,7 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
 		break;
 	}
 
+hypercall_complete:
 	return kvm_hv_hypercall_complete(vcpu, ret);
 }