On Fri, Apr 06, 2018 at 07:54:02PM +0300, Yury Norov wrote: > In general, kick_all_cpus_sync() is needed to switch contexts. But exit from > userspace is anyway the switch of context. And while in userspace, we cannot > do something wrong on kernel side. For me it means that we can safely drop > IPI for all userspace modes - both normal and nohz_full. This *may* be true, but only if we never have to patch text in the windows: * between exception entry and eqs_exit() * between eqs_enter() and exception return * between eqs_enter() and eqs_exit() in the idle loop. If it's possible that we need to execute patched text in any of those paths, we must IPI all CPUs in order to correctly serialize things. Digging a bit, I also thing that our ct_user_exit and ct_user_enter usage is on dodgy ground today. For example, in el0_dbg we call do_debug_exception() *before* calling ct_user_exit. Which I believe means we'd use RCU while supposedly in an extended quiescent period, which would be bad. In other paths, we unmask all DAIF bits before calling ct_user_exit, so we could similarly take an EL1 debug exception without having exited the extended quiescent period. I think similar applies to SDEI; we don't negotiate with RCU prior to invoking handlers, which might need RCU. > If it's correct, for v3 I would suggest: > - in kick_all_cpus_sync() mask all is_idle_task() and user_mode() CPUs; > - add isb() for arm64 in do_idle() path only - this path doesn't imply > context switch. As mentioned in my other reply, I don't think the ISB in do_idle() makes sense, unless that occurs *after* we exit the extended quiescent state. Thanks, Mark. -- To unsubscribe from this list: send the line "unsubscribe kvm-ppc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
