Il 30/05/2013 15:10, Gleb Natapov ha scritto: > On Thu, May 30, 2013 at 02:58:09PM +0200, Paolo Bonzini wrote: >> Il 30/05/2013 14:34, Gleb Natapov ha scritto: >>>>>>> >>>>>>> Ah, we check kvm_apic_has_events() in runnable. Then yes, we will not >>>>>>> lose the event. >>>>> >>>>> Ok, then I'd prefer to have the cmpxchg directly in the if, as in >>>>> http://article.gmane.org/gmane.comp.emulators.kvm.devel/110505 >>>>> >>> I still do not. Both of them are tricky, mine does not coalesce events >>> needlessly. >> >> Agreed that both are tricky, but I don't think my patch is coalescing >> events. If you have >> >> INIT SIPI INIT SIPI >> ^ ^ >> INIT bit cleared here SIPI bit checked here >> > Not sure I understand what you are trying to say here. I'll redo the picture below. >> my patch KVM sees apic_events = INIT | SIPI and deduces that the SIPI >> bit was set by the second SIPI, not by the first. In fact the first >> SIPI was cancelled by the second INIT, and thus should not be processed >> at all. > That is called coalesced. Coalescing would be something like INIT SIPI SIPI -> INIT SIPI. This is not coalescing, it is proper detection of a cancelled SIPI. We have: event sent event processed pending_events INIT INIT SIPI INIT|SIPI INIT SIPI XX INIT INIT SIPI INIT|SIPI YY SIPI INIT|SIPI failed cmpxchg INIT|SIPI INIT SIPI SIPI 0 At the line I marked with YY, you're processing an interrupt that is not anymore in the pending_events. It was dropped at point XX. With my patch it is: event sent event processed pending_events INIT INIT SIPI INIT|SIPI INIT SIPI XX INIT INIT SIPI INIT|SIPI failed cmpxchg INIT|SIPI INIT SIPI SIPI 0 >> Instead, with your patch KVM will service all four events; strictly >> speaking it is wrong to service the first SIPI, which is why I prefer >> having the cmpxchg in the beginning. > > Why is it wrong? Because the first SIPI was dropped atomically with the triggering of the second INIT, it's as if you were handling it twice. Paolo -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
