Marcelo Tosatti wrote: > On Mon, Oct 12, 2009 at 11:07:28AM +0200, Jan Kiszka wrote: >> Avi Kivity wrote: >>> On 10/12/2009 09:50 AM, Jan Kiszka wrote: >>>>> Apic is lockless. For ioapic/pic I used spinlocks initially, but Avi >>>>> prefers mutexes. Theoretically it is possible to make them lockless, >>>>> but code will be complex and eventually more slow, since more then two >>>>> atomic operation will be used on irq injection path. >>>>> >>>> Well, lockless is another thing. >>>> >>>> But also converting to spinlocks would indeed add some overhead: >>>> irqsave/restore. But I wonder if this isn't worth it, at least when >>>> looking at the (supposed to be fast) device passthrough scenario which >>>> would be simpler and faster. >>>> >>> I'm worried about disabling irqs for non-device-assignment cases. It >>> would be more palatable if ioapic was completely O(1) (there are some >>> per-vcpu loops in there, shouldn't be too bad for 16 vcpus, but we want >>> to scale). >> Yeah, what a pity. That's likely not solvable in a generic way, given >> that the guest finally decided how many VCPUs may listen to a line. >> >> OK, but dropping interrupt_work from the MSI path is still worthwhile, >> and probably more future-proof anyway. > > Seems appropriate to convert the process context work to threaded > interrupt (instead of workqueue). That should help latency. > Not for the trivial case (I want to avoid scheduling as far as possible). Jan
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