Gleb Natapov wrote: > On Wed, Oct 21, 2009 at 11:34:51AM -0400, Gregory Haskins wrote: >> Gleb Natapov wrote: >>> On Wed, Oct 21, 2009 at 10:34:53AM -0400, Gregory Haskins wrote: >>>> IRQFD currently uses a deferred workqueue item to execute the injection >>>> operation. It was originally designed this way because kvm_set_irq() >>>> required the caller to hold the irq_lock mutex, and the eventfd callback >>>> is invoked from within a non-preemptible critical section. >>>> >>>> With the advent of lockless injection support in kvm_set_irq, the deferment >>>> mechanism is no longer technically needed. Since context switching to the >>>> workqueue is a source of interrupt latency, lets switch to a direct >>>> method. >>>> >>> kvm_set_irq is fully lockless only in MSI case. IOAPIC/PIC has mutexes. >> Right, but irqfd by design only works with MSI (or MSI like edge >> triggers) anyway. Legacy-type injections follow a different path. >> > Ah, If this the case and it will stay that way then the change looks OK > to me. I believe Avi, Michael, et. al. were in agreement with me on that design choice. I believe the reason is that there is no good way to do EOI/ACK feedback within the constraints of an eventfd pipe which would be required for the legacy pin-type interrupts. Therefore, we won't even bother trying. High-performance subsystems will use irqfd/msi, and legacy emulation can use the existing injection code (which includes the necessary feedback for ack/eoi). To that point, perhaps it should be better documented. If we need a v2, I will add a comment. Kind Regards, -Greg
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