Hi Paul, During the study of RCU, I encountered confusion about how X86's irq_disable affects Linux IPI delivery. So I spent a week trying to find out the fundamental mechanism by debugging the qemu virtual machine. I think my experience may help others, and I guess the perfbook mailing list is the best place to share my experience. Please correct me if it is improper. I learn how Linux IPI is blocked by irq_disable by means of debug qemu virtual machine: Fig 1 (https://ibb.co/Ypv3hNm) CPU 1 sending IPI; Fig 2 (https://ibb.co/162sVyV) CPU 0 receive the interrupt; Fig 3- Fig4 (https://ibb.co/bzFkckM, https://ibb.co/TmC3rn4) but CPU 0 has irq disabled, so the IPI will not be served at that moment; Fig 5 (https://ibb.co/k8BpBMq) CPU 0 enable irq; Fig 6 (https://ibb.co/zNBrMJk) now IPI got served Thanks for your continuous encouragement ;-) Thanks Zhouyi
