KVM: nVMX: Treat eVMCS as enabled for guest iff Hyper-V is also enabled

When querying whether or not eVMCS is enabled on behalf of the guest,
treat eVMCS as enable if and only if Hyper-V is enabled/exposed to the
guest.

Note, flows that come from the host, e.g. KVM_SET_NESTED_STATE, must NOT
check for Hyper-V being enabled as KVM doesn't require guest CPUID to be
set before most ioctls().
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20220830133737.1539624-7-vkuznets@redhat.comSigned-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/evmcs.c

@@ -334,6 +334,9 @@ uint16_t nested_get_evmcs_version(struct kvm_vcpu *vcpu)
 	 * versions: lower 8 bits is the minimal version, higher 8 bits is the
 	 * maximum supported version. KVM supports versions from 1 to
 	 * KVM_EVMCS_VERSION.
+	 *
+	 * Note, do not check the Hyper-V is fully enabled in guest CPUID, this
+	 * helper is used to _get_ the vCPU's supported CPUID.
 	 */
 	if (kvm_cpu_cap_get(X86_FEATURE_VMX) &&
 	    (!vcpu || to_vmx(vcpu)->nested.enlightened_vmcs_enabled))

arch/x86/kvm/vmx/nested.c

@@ -1982,7 +1982,7 @@ static enum nested_evmptrld_status nested_vmx_handle_enlightened_vmptrld(
 	bool evmcs_gpa_changed = false;
 	u64 evmcs_gpa;
 
-	if (likely(!vmx->nested.enlightened_vmcs_enabled))
+	if (likely(!guest_cpuid_has_evmcs(vcpu)))
 		return EVMPTRLD_DISABLED;
 
 	if (!nested_enlightened_vmentry(vcpu, &evmcs_gpa)) {
@@ -2863,7 +2863,7 @@ static int nested_vmx_check_controls(struct kvm_vcpu *vcpu,
 	    nested_check_vm_entry_controls(vcpu, vmcs12))
 		return -EINVAL;
 
-	if (to_vmx(vcpu)->nested.enlightened_vmcs_enabled)
+	if (guest_cpuid_has_evmcs(vcpu))
 		return nested_evmcs_check_controls(vmcs12);
 
 	return 0;
@@ -3145,7 +3145,7 @@ static bool nested_get_evmcs_page(struct kvm_vcpu *vcpu)
 	 * L2 was running), map it here to make sure vmcs12 changes are
 	 * properly reflected.
 	 */
-	if (vmx->nested.enlightened_vmcs_enabled &&
+	if (guest_cpuid_has_evmcs(vcpu) &&
 	    vmx->nested.hv_evmcs_vmptr == EVMPTR_MAP_PENDING) {
 		enum nested_evmptrld_status evmptrld_status =
 			nested_vmx_handle_enlightened_vmptrld(vcpu, false);
@@ -5077,7 +5077,7 @@ static int handle_vmclear(struct kvm_vcpu *vcpu)
 	 * state. It is possible that the area will stay mapped as
 	 * vmx->nested.hv_evmcs but this shouldn't be a problem.
 	 */
-	if (likely(!vmx->nested.enlightened_vmcs_enabled ||
+	if (likely(!guest_cpuid_has_evmcs(vcpu) ||
 		   !nested_enlightened_vmentry(vcpu, &evmcs_gpa))) {
 		if (vmptr == vmx->nested.current_vmptr)
 			nested_release_vmcs12(vcpu);

arch/x86/kvm/vmx/vmx.c

@@ -1930,8 +1930,7 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		 * sanity checking and refuse to boot. Filter all unsupported
 		 * features out.
 		 */
-		if (!msr_info->host_initiated &&
-		    vmx->nested.enlightened_vmcs_enabled)
+		if (!msr_info->host_initiated && guest_cpuid_has_evmcs(vcpu))
 			nested_evmcs_filter_control_msr(msr_info->index,
 							&msr_info->data);
 		break;

arch/x86/kvm/vmx/vmx.h

@@ -626,4 +626,14 @@ static inline bool vmx_can_use_ipiv(struct kvm_vcpu *vcpu)
 	return  lapic_in_kernel(vcpu) && enable_ipiv;
 }
 
+static inline bool guest_cpuid_has_evmcs(struct kvm_vcpu *vcpu)
+{
+	/*
+	 * eVMCS is exposed to the guest if Hyper-V is enabled in CPUID and
+	 * eVMCS has been explicitly enabled by userspace.
+	 */
+	return vcpu->arch.hyperv_enabled &&
+	       to_vmx(vcpu)->nested.enlightened_vmcs_enabled;
+}
+
 #endif /* __KVM_X86_VMX_H */