TB_NIL
vcore->preempt_tb = TB_NIL;
vcpu->arch.busy_preempt = TB_NIL;
if (vc->preempt_tb != TB_NIL) {
vc->preempt_tb = TB_NIL;
if (vcpu->arch.busy_preempt != TB_NIL) {
vcpu->arch.busy_preempt = TB_NIL;
vcpu->arch.busy_preempt != TB_NIL) {
vcpu->arch.busy_preempt = TB_NIL;
vc->preempt_tb = TB_NIL;
vcpu->arch.busy_preempt = TB_NIL;
vc->preempt_tb != TB_NIL)