KVM: nVMX: Reset eVMCS clean fields data from prepare_vmcs02()

When nested state migration happens during L1's execution, it
is incorrect to modify eVMCS as it is L1 who 'owns' it at the moment.
At least genuine Hyper-V seems to not be very happy when 'clean fields'
data changes underneath it.

'Clean fields' data is used in KVM twice: by copy_enlightened_to_vmcs12()
and prepare_vmcs02_rare() so we can reset it from prepare_vmcs02() instead.

While at it, update a comment stating why exactly we need to reset
'hv_clean_fields' data from L0.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210526132026.270394-10-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/nested.c

@@ -2059,14 +2059,10 @@ void nested_sync_vmcs12_to_shadow(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
 
-	if (evmptr_is_valid(vmx->nested.hv_evmcs_vmptr)) {
+	if (evmptr_is_valid(vmx->nested.hv_evmcs_vmptr))
 		copy_vmcs12_to_enlightened(vmx);
-		/* All fields are clean */
-		vmx->nested.hv_evmcs->hv_clean_fields |=
-			HV_VMX_ENLIGHTENED_CLEAN_FIELD_ALL;
-	} else {
+	else
 		copy_vmcs12_to_shadow(vmx);
-	}
 
 	vmx->nested.need_vmcs12_to_shadow_sync = false;
 }
@@ -2616,6 +2612,17 @@ static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
 
 	kvm_rsp_write(vcpu, vmcs12->guest_rsp);
 	kvm_rip_write(vcpu, vmcs12->guest_rip);
+
+	/*
+	 * It was observed that genuine Hyper-V running in L1 doesn't reset
+	 * 'hv_clean_fields' by itself, it only sets the corresponding dirty
+	 * bits when it changes a field in eVMCS. Mark all fields as clean
+	 * here.
+	 */
+	if (evmptr_is_valid(vmx->nested.hv_evmcs_vmptr))
+		vmx->nested.hv_evmcs->hv_clean_fields |=
+			HV_VMX_ENLIGHTENED_CLEAN_FIELD_ALL;
+
 	return 0;
 }