KVM: nVMX: Update vmcs12 for MSR_IA32_DEBUGCTLMSR when it's written

KVM unconditionally intercepts WRMSR to MSR_IA32_DEBUGCTLMSR.  In the
unlikely event that L1 allows L2 to write L1's MSR_IA32_DEBUGCTLMSR, but
but saves L2's value on VM-Exit, update vmcs12 during L2's WRMSR so as
to eliminate the need to VMREAD the value from vmcs02 on nested VM-Exit.
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/nested.c

@@ -3563,10 +3563,8 @@ static void sync_vmcs02_to_vmcs12(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12)
 		(vmcs12->vm_entry_controls & ~VM_ENTRY_IA32E_MODE) |
 		(vm_entry_controls_get(to_vmx(vcpu)) & VM_ENTRY_IA32E_MODE);
 
-	if (vmcs12->vm_exit_controls & VM_EXIT_SAVE_DEBUG_CONTROLS) {
+	if (vmcs12->vm_exit_controls & VM_EXIT_SAVE_DEBUG_CONTROLS)
 		kvm_get_dr(vcpu, 7, (unsigned long *)&vmcs12->guest_dr7);
-		vmcs12->guest_ia32_debugctl = vmcs_read64(GUEST_IA32_DEBUGCTL);
-	}
 
 	if (vmcs12->vm_exit_controls & VM_EXIT_SAVE_IA32_EFER)
 		vmcs12->guest_ia32_efer = vcpu->arch.efer;

arch/x86/kvm/vmx/vmx.c

@@ -1845,6 +1845,14 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 			get_vmcs12(vcpu)->guest_sysenter_esp = data;
 		vmcs_writel(GUEST_SYSENTER_ESP, data);
 		break;
+	case MSR_IA32_DEBUGCTLMSR:
+		if (is_guest_mode(vcpu) && get_vmcs12(vcpu)->vm_exit_controls &
+						VM_EXIT_SAVE_DEBUG_CONTROLS)
+			get_vmcs12(vcpu)->guest_ia32_debugctl = data;
+
+		ret = kvm_set_msr_common(vcpu, msr_info);
+		break;
+
 	case MSR_IA32_BNDCFGS:
 		if (!kvm_mpx_supported() ||
 		    (!msr_info->host_initiated &&