KVM: nVMX: Update vmcs12 for SYSENTER MSRs when they're written

For L2, KVM always intercepts WRMSR to SYSENTER MSRs.  Update vmcs12 in
the WRMSR handler so that they don't need to be (re)read from vmcs02 on
every 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

@@ -3521,6 +3521,10 @@ static void sync_vmcs02_to_vmcs12(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12)
 	vmcs12->guest_cs_ar_bytes = vmcs_read32(GUEST_CS_AR_BYTES);
 	vmcs12->guest_ss_ar_bytes = vmcs_read32(GUEST_SS_AR_BYTES);
 
+	vmcs12->guest_sysenter_cs = vmcs_read32(GUEST_SYSENTER_CS);
+	vmcs12->guest_sysenter_esp = vmcs_readl(GUEST_SYSENTER_ESP);
+	vmcs12->guest_sysenter_eip = vmcs_readl(GUEST_SYSENTER_EIP);
+
 	vmcs12->guest_interruptibility_info =
 		vmcs_read32(GUEST_INTERRUPTIBILITY_INFO);
 
@@ -3566,9 +3570,6 @@ static void sync_vmcs02_to_vmcs12(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12)
 
 	if (vmcs12->vm_exit_controls & VM_EXIT_SAVE_IA32_EFER)
 		vmcs12->guest_ia32_efer = vcpu->arch.efer;
-	vmcs12->guest_sysenter_cs = vmcs_read32(GUEST_SYSENTER_CS);
-	vmcs12->guest_sysenter_esp = vmcs_readl(GUEST_SYSENTER_ESP);
-	vmcs12->guest_sysenter_eip = vmcs_readl(GUEST_SYSENTER_EIP);
 }
 
 /*

arch/x86/kvm/vmx/vmx.c

@@ -1831,12 +1831,18 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		break;
 #endif
 	case MSR_IA32_SYSENTER_CS:
+		if (is_guest_mode(vcpu))
+			get_vmcs12(vcpu)->guest_sysenter_cs = data;
 		vmcs_write32(GUEST_SYSENTER_CS, data);
 		break;
 	case MSR_IA32_SYSENTER_EIP:
+		if (is_guest_mode(vcpu))
+			get_vmcs12(vcpu)->guest_sysenter_eip = data;
 		vmcs_writel(GUEST_SYSENTER_EIP, data);
 		break;
 	case MSR_IA32_SYSENTER_ESP:
+		if (is_guest_mode(vcpu))
+			get_vmcs12(vcpu)->guest_sysenter_esp = data;
 		vmcs_writel(GUEST_SYSENTER_ESP, data);
 		break;
 	case MSR_IA32_BNDCFGS: