On Thu, 2012-07-12 at 12:35 +0300, Avi Kivity wrote: > On 07/11/2012 10:57 PM, Alex Williamson wrote: > >> > >> > We still have classic KVM device assignment to provide fast-path INTx. > >> > But if we want to replace it midterm, I think it's necessary for VFIO to > >> > be able to provide such a path as well. > >> > >> I would like VFIO to have no regressions vs. kvm device assignment, > >> except perhaps in uncommon corner cases. So I agree. > > > > I ran a few TCP_RR netperf tests forcing a 1Gb tg3 nic to use INTx. > > Without irqchip support vfio gets a bit more than 60% of KVM device > > assignment. That's a little bit of an unfair comparison since it's more > > than just the I/O path. With the proposed interfaces here, enabling > > irqchip, vfio is within 10% of KVM device assignment for INTx. For MSI, > > I can actually make vfio come out more than 30% better than KVM device > > assignment if I send the eventfd from the hard irq handler. Using a > > threaded handler as the code currently does, vfio is still behind KVM. > > It's hard to beat a direct call chain. > > We can have a direct call chain with vfio too, using a custom eventfd > poll function, no? Assuming we set up a fast path for unicast msi. You'll have to help me out a little, eventfd_signal walks the wait_queue and calls each function. On the injection path that includes irqfd_wakeup. For an MSI that seems to already provide direct injection. For level we'll schedule_work, so that explains the overhead in that path, but it's not too dissimilar to a a threaded irq. vfio does something very similar, so there's a schedule_work both on inject and on eoi. I'll have to check whether anything prevents the unmask from the wait_queue function in vfio, that could be a significant chunk of the gap. Where's the custom poll function come into play? Thanks, Alex -- 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
