Excerpts from Mathieu Desnoyers's message of July 9, 2020 12:12 am: > ----- On Jul 8, 2020, at 1:17 AM, Nicholas Piggin npig...@gmail.com wrote: > >> Excerpts from Mathieu Desnoyers's message of July 7, 2020 9:25 pm: >>> ----- On Jul 7, 2020, at 1:50 AM, Nicholas Piggin npig...@gmail.com wrote: >>> > [...] >>>> I should actually change the comment for 64-bit because soft masked >>>> interrupt replay is an interesting case. I thought it was okay (because >>>> the IPI would cause a hard interrupt which does do the rfi) but that >>>> should at least be written. >>> >>> Yes. >>> >>>> The context synchronisation happens before >>>> the Linux IPI function is called, but for the purpose of membarrier I >>>> think that is okay (the membarrier just needs to have caused a memory >>>> barrier + context synchronistaion by the time it has done). >>> >>> Can you point me to the code implementing this logic ? >> >> It's mostly in arch/powerpc/kernel/exception-64s.S and >> powerpc/kernel/irq.c, but a lot of asm so easier to explain. >> >> When any Linux code does local_irq_disable(), we set interrupts as >> software-masked in a per-cpu flag. When interrupts (including IPIs) come >> in, the first thing we do is check that flag and if we are masked, then >> record that the interrupt needs to be "replayed" in another per-cpu >> flag. The interrupt handler then exits back using RFI (which is context >> synchronising the CPU). Later, when the kernel code does >> local_irq_enable(), it checks the replay flag to see if anything needs >> to be done. At that point we basically just call the interrupt handler >> code like a normal function, and when that returns there is no context >> synchronising instruction. > > AFAIU this can only happen for interrupts nesting over irqoff sections, > therefore over kernel code, never userspace, right ?
Right. >> So membarrier IPI will always cause target CPUs to perform a context >> synchronising instruction, but sometimes it happens before the IPI >> handler function runs. > > If my understanding is correct, the replayed interrupt handler logic > only nests over kernel code, which will eventually need to issue a > context synchronizing instruction before returning to user-space. Yes. > All we care about is that starting from the membarrier, each core > either: > > - interrupt user-space to issue the context synchronizing instruction if > they were running userspace, or > - _eventually_ issue a context synchronizing instruction before returning > to user-space if they were running kernel code. > > So your earlier statement "the membarrier just needs to have caused a memory > barrier + context synchronistaion by the time it has done" is not strictly > correct: the context synchronizing instruction does not strictly need to > happen on each core before membarrier returns. A similar line of thoughts > can be followed for memory barriers. Ah okay that makes it simpler, then no such speical comment is required for the powerpc specific interrupt handling. Thanks, Nick
