2015-03-18 15:37-0400, Bandan Das: > Radim Krčmář <rkrcmar@xxxxxxxxxx> writes: > > kvm_ioapic_update_eoi() wasn't called if directed EOI was enabled. > > We need to do that for irq notifiers. (Like with edge interrupts.) > > Wow! It's interesting that this path is only hit with Xen as guest. Linux doesn't use directed EOI ... KVM should fail with anything that depends on PIT, so probably only Xen bothered to implement it :) > I always thought of directed EOI as a "security feature" since broadcast > could lead to interrupt storms (or something like that) :) I think it is just an unpopular optimization for large systems. (With multiple IO-APICs: IRQ handler knows which ones need the EOI, but LAPIC doesn't, hence we avoid some useless poking if OS does it ... EOI interrupt storm happens because right after EOI, the IO-APIC can send another interrupt and real hardware is slow, so CPU manages some cycles before receiving the next one, but KVM works instantaneously.) -- 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
