On 27/01/25 12:17, Valentin Schneider wrote: > On 22/01/25 01:22, Frederic Weisbecker wrote: >> And NMIs interrupting userspace don't call >> enter_from_user_mode(). In fact they don't call irqentry_enter_from_user_mode() >> like regular IRQs but irqentry_nmi_enter() instead. Well that's for archs >> implementing common entry code, I can't speak for the others. >> > > That I didn't realize, so thank you for pointing it out. Having another > look now, I mistook DEFINE_IDTENTRY_RAW(exc_int3) for the general case > when it really isn't :( > >> Unifying the behaviour between user and idle such that the IRQs/NMIs exit the >> CT_STATE can be interesting but I fear this may not come for free. You would >> need to save the old state on IRQ/NMI entry and restore it on exit. >> > > That's what I tried to avoid, but it sounds like there's no nice way around it. > >> Do we really need it? >> > > Well, my problem with not doing IDLE->KERNEL transitions on IRQ/NMI is that > this leads the IPI deferral logic to observe a technically-out-of-sync sate > for remote CPUs. Consider: > > CPUx CPUy > state := CT_STATE_IDLE > ... > ~>IRQ > ... > ct_nmi_enter() > [in the kernel proper by now] > > text_poke_bp_batch() > ct_set_cpu_work(CPUy, CT_WORK_SYNC) > READ CPUy ct->state > `-> CT_IDLE_STATE > `-> defer IPI > > > I thought this meant I would need to throw out the "defer IPIs if CPU is > idle" part, but AIUI this also affects CT_STATE_USER and CT_STATE_GUEST, > which is a bummer :( Soooo I've been thinking... Isn't (context_tracking.state & CT_RCU_WATCHING) pretty much a proxy for knowing whether a CPU is executing in kernelspace, including NMIs? NMI interrupts userspace/VM/idle -> ct_nmi_enter() -> it becomes true IRQ interrupts idle -> ct_irq_enter() -> it becomes true IRQ interrupts userspace -> __ct_user_exit() -> it becomes true IRQ interrupts VM -> __ct_user_exit() -> it becomes true IOW, if I gate setting deferred work by checking for this instead of explicitely CT_STATE_KERNEL, "it should work" and prevent the aforementioned issue? Or should I be out drinking instead? :-)
