KVM: LAPIC: Keep stored TMCCT register value 0 after KVM_SET_LAPIC

KVM_GET_LAPIC stores the current value of TMCCT and KVM_SET_LAPIC's memcpy
stores it in vcpu->arch.apic->regs, KVM_SET_LAPIC could store zero in
vcpu->arch.apic->regs after it uses it, and then the stored value would
always be zero. In addition, the TMCCT is always computed on-demand and
never directly readable.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
Message-Id: <1623223000-18116-1-git-send-email-wanpengli@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/lapic.c

@@ -2631,6 +2631,7 @@ int kvm_apic_set_state(struct kvm_vcpu *vcpu, struct kvm_lapic_state *s)
 	apic_manage_nmi_watchdog(apic, kvm_lapic_get_reg(apic, APIC_LVT0));
 	update_divide_count(apic);
 	__start_apic_timer(apic, APIC_TMCCT);
+	kvm_lapic_set_reg(apic, APIC_TMCCT, 0);
 	kvm_apic_update_apicv(vcpu);
 	apic->highest_isr_cache = -1;
 	if (vcpu->arch.apicv_active) {