On Mon, Jan 11, 2010 at 11:36:16AM +0200, Gleb Natapov wrote: > On Mon, Jan 11, 2010 at 11:19:19AM +0200, Michael S. Tsirkin wrote: > > On Mon, Jan 11, 2010 at 11:08:38AM +0200, Gleb Natapov wrote: > > > On Mon, Jan 11, 2010 at 11:02:27AM +0200, Michael S. Tsirkin wrote: > > > > On Mon, Jan 11, 2010 at 11:01:27AM +0200, Gleb Natapov wrote: > > > > > On Thu, Jan 07, 2010 at 12:36:01PM +0200, Michael S. Tsirkin wrote: > > > > > > Or if I do it the other way: > > > > > > remove_wait_queue(irqfd->wqh, &irqfd->wait); > > > > > > -> > > > > > > eventfd_read_ctx(irqfd->eventfd, &ucnt); > > > > > > > > > > > > now, if device signals eventfd at point marked by ->, > > > > > > it will not be sent but counter will be cleared, > > > > > > so we will loose a message. > > > > > > > > > > > May be I am missing something here, but why doing it like that is a > > > > > problem? Event delivery races with interrupt masking so making masking > > > > > succeed before event delivery is OK. Event generation is asynchronous > > > > > anyway and could have happened jiffy latter anyway. > > > > > > > > > > -- > > > > > Gleb. > > > > > > > > No, event generation would only trigger a single interrupt. This race > > > > generates two interrupts for a single event. This can never happen with > > > > real hardware. eventfd_ctx_remove_wait_queue would solve this problem. > > > > > > > In quoted test above you are saying that "we will loose a message". So > > > how does it generates two interrupts when we loose a message? > > > > > > -- > > > Gleb. > > > > > > Right, sorry. I think what you miss is the fact that this is done during > > interrupt vector masking/unmasking, so events signalled while eventfd is not > > assigned to interrupt must not be lost, they should be pending and > > delivered later when interrupt vector is unmasked, that is when > > eventfd is reassigned to an interrupt. > > > Is this how MSI works? If interrupt is triggered while it is masked it > is reasserted after unmasking? This is certainly no true for interrupt > masking on irq chip level. Yes. > > So this implementation really loses an interrupt: > > > > remove_wait_queue(irqfd->wqh, &irqfd->wait); > > -> at this point vector is masked: we are not polling > > eventfd anymore, event triggered at this point should cause interrupt > > after vector is unmasked, but the only thing is causes > > is counter increment in eventfd. > > > > eventfd_read_ctx(irqfd->eventfd, &ucnt); > > -> the above call would clear the counter, so > > we won't get an interrupt when vector is later > > unmasked. > > > Don't you going to use ucnt to set interrupt status bit? Can't you > re-trigger the interrupt after unmasking if status bit is set? Yes, this is what Davidel suggested originally. There's nowhere to save the ucnt though because event is being deasserted. > -- > Gleb. -- 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
