On Thu, 2015-05-14 at 17:46 +0200, Paolo Bonzini wrote: > > On 14/05/2015 17:23, Alex Williamson wrote: > > > Would irq_ack_notifiers be used at all with this patch set? Resampling > > > of IOAPIC level-triggered interrupts would be implemented in userspace. > > > For the same reason, assigned devices using legacy device assignment > > > probably would not be able to use INTX (so this feature should depend on > > > !KVM_DEVICE_ASSIGNMENT). Add the emulated i8254 and the last user of > > > irq_ack_notifiers goes away. > > > > > > Alex, how does VFIO do INTX resampling if you're using TCG or -machine > > > kernel_irqchip=off? (Context: this series keeps the local APIC > > > emulation in the kernel, thus including MSI, but moves the IOAPIC > > > emualtion to userspace). > > > > Without KVM irqchip, we use a rudimentary approach where we disable > > mmaps to the device when an interrupt occurs. This makes us trap all > > accesses to the device. We then handle any device access from the guest > > as a potential EOI and unmask the interrupt. If the interrupt re-fires, > > we've at least rate-limited it. If it doesn't, a timer eventually > > re-enables devices mmaps. It's not very efficient, but it works (for > > both PCI and platform devices) and avoids IRQ APIs through QEMU that get > > very platform/machine specific. Thanks, > > If we move the IOAPIC back to userspace, it probably would be easier to > implement irqfd/resamplefd in userspace as well. Let the bikeshedding > start! The current solution is merely functional with the benefit of being architecture and machine-type independent. If someone actually cared about vfio assigned device performance without KVM irqchip, we'd certainly need a more deterministic and efficient EOI path. 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
