* Anthony Liguori <aliguori@xxxxxxxxxx> wrote: > When using PCI LNK interrupts, the ISR flag serves two purposes. It > indicates that an interrupt was raised (since the actual interrupt line may > be shared) and it is used to acknowledge the interrupt (since PCI LNK lines > are level triggered). ok. > It seems like this patch is simply avoiding raising the interrupt line if the > ISR has not been acknowledged yet. I don't think there's a functional issue > here [...] Thanks for confirming this - so i think we still do not understand the root cause of the ping latency and why this change fixed it ... > [...] but I'm surprised that it's a win. There should be a very short window > when the interrupt is lowered in the APIC but still not acknowledged in the > ISR. > > You should just be saving a pretty cheap system call. I wonder if the system > call is taking longer than it should.. Well the optimization also avoids unnecessary VM exits (due to the injection, which interrupts a guest context immediately, even if it's running on another CPU), not just system calls - so it could be more expensive than a system call, right? Thanks, Ingo -- 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
