KVM: x86: hyper-v: Introduce KVM_CAP_HYPERV_ENFORCE_CPUID

Modeled after KVM_CAP_ENFORCE_PV_FEATURE_CPUID, the new capability allows
for limiting Hyper-V features to those exposed to the guest in Hyper-V
CPUIDs (0x40000003, 0x40000004, ...).
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210521095204.2161214-3-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Documentation/virt/kvm/api.rst

@@ -6891,3 +6891,14 @@ This capability is always enabled.
 This capability indicates that the KVM virtual PTP service is
 supported in the host. A VMM can check whether the service is
 available to the guest on migration.
+
+8.33 KVM_CAP_HYPERV_ENFORCE_CPUID
+-----------------------------
+
+Architectures: x86
+
+When enabled, KVM will disable emulated Hyper-V features provided to the
+guest according to the bits Hyper-V CPUID feature leaves. Otherwise, all
+currently implmented Hyper-V features are provided unconditionally when
+Hyper-V identification is set in the HYPERV_CPUID_INTERFACE (0x40000001)
+leaf.

arch/x86/include/asm/kvm_host.h

@@ -543,6 +543,7 @@ struct kvm_vcpu_hv {
 	struct kvm_vcpu_hv_stimer stimer[HV_SYNIC_STIMER_COUNT];
 	DECLARE_BITMAP(stimer_pending_bitmap, HV_SYNIC_STIMER_COUNT);
 	cpumask_t tlb_flush;
+	bool enforce_cpuid;
 };
 
 /* Xen HVM per vcpu emulation context */

arch/x86/kvm/hyperv.c

@@ -1853,6 +1853,27 @@ void kvm_hv_set_cpuid(struct kvm_vcpu *vcpu)
 		vcpu->arch.hyperv_enabled = false;
 }
 
+int kvm_hv_set_enforce_cpuid(struct kvm_vcpu *vcpu, bool enforce)
+{
+	struct kvm_vcpu_hv *hv_vcpu;
+	int ret = 0;
+
+	if (!to_hv_vcpu(vcpu)) {
+		if (enforce) {
+			ret = kvm_hv_vcpu_init(vcpu);
+			if (ret)
+				return ret;
+		} else {
+			return 0;
+		}
+	}
+
+	hv_vcpu = to_hv_vcpu(vcpu);
+	hv_vcpu->enforce_cpuid = enforce;
+
+	return ret;
+}
+
 bool kvm_hv_hypercall_enabled(struct kvm_vcpu *vcpu)
 {
 	return vcpu->arch.hyperv_enabled && to_kvm_hv(vcpu->kvm)->hv_guest_os_id;

arch/x86/kvm/hyperv.h

@@ -138,6 +138,7 @@ void kvm_hv_invalidate_tsc_page(struct kvm *kvm);
 void kvm_hv_init_vm(struct kvm *kvm);
 void kvm_hv_destroy_vm(struct kvm *kvm);
 void kvm_hv_set_cpuid(struct kvm_vcpu *vcpu);
+int kvm_hv_set_enforce_cpuid(struct kvm_vcpu *vcpu, bool enforce);
 int kvm_vm_ioctl_hv_eventfd(struct kvm *kvm, struct kvm_hyperv_eventfd *args);
 int kvm_get_hv_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid2 *cpuid,
 		     struct kvm_cpuid_entry2 __user *entries);

arch/x86/kvm/x86.c

@@ -3955,6 +3955,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 	case KVM_CAP_HYPERV_TLBFLUSH:
 	case KVM_CAP_HYPERV_SEND_IPI:
 	case KVM_CAP_HYPERV_CPUID:
+	case KVM_CAP_HYPERV_ENFORCE_CPUID:
 	case KVM_CAP_SYS_HYPERV_CPUID:
 	case KVM_CAP_PCI_SEGMENT:
 	case KVM_CAP_DEBUGREGS:
@@ -4878,6 +4879,9 @@ static int kvm_vcpu_ioctl_enable_cap(struct kvm_vcpu *vcpu,
 
 		return static_call(kvm_x86_enable_direct_tlbflush)(vcpu);
 
+	case KVM_CAP_HYPERV_ENFORCE_CPUID:
+		return kvm_hv_set_enforce_cpuid(vcpu, cap->args[0]);
+
 	case KVM_CAP_ENFORCE_PV_FEATURE_CPUID:
 		vcpu->arch.pv_cpuid.enforce = cap->args[0];
 		if (vcpu->arch.pv_cpuid.enforce)

include/uapi/linux/kvm.h

@@ -1083,6 +1083,7 @@ struct kvm_ppc_resize_hpt {
 #define KVM_CAP_SGX_ATTRIBUTE 196
 #define KVM_CAP_VM_COPY_ENC_CONTEXT_FROM 197
 #define KVM_CAP_PTP_KVM 198
+#define KVM_CAP_HYPERV_ENFORCE_CPUID 199
 
 #ifdef KVM_CAP_IRQ_ROUTING