On 06.03.2013, at 15:11, Gleb Natapov wrote: > On Wed, Mar 06, 2013 at 02:41:04PM +0100, Paolo Bonzini wrote: >> Il 06/03/2013 14:14, Gleb Natapov ha scritto: >>>>>> The problem would only start if KVM_SET_IRQCHIP_TYPE (new name of >>>>>> KVM_CREATE_IRQCHIP_ARGS) forced you to later call KVM_CREATE_DEVICE. >>>>> >>>>> Ah, I see. I don't see why it would. The fact that there is a >>>>> "LAPIC" doesn't mean that the per-vcpu SET_INTERRUPT ioctl stops >>>>> working. So if SET_IRQCHIP_TYPE(!none) breaks user-space interrupt >>>>> controller emulation I would consider that a bug. >>>> >>> For x86 this is the case though. I do not see how it can't be. If >>> LAPIC is emulated in userspace SET_INTERRUPT is used to pass IRQ >>> vector that should be handled as a result of LAPIC emulation. >> >> SET_IRQCHIP_TYPE creates the LAPICs; it would indeed break userspace >> LAPIC emulation because the LAPICs would not cause userspace exits anymore. >> > The reason it will break userspace is much more fundamental than that. > There is not interface suitable for communication between userspace > PIC/IOAPIC and in-kernel LAPIC right now. > > Why LAPICs should cause userspace exit? The reason we exit to userspace > without in kernel irqchip is because we need LAPIC emulation to run. > >> However, it need not mandate the usage of an in-kernel IOAPIC or PIC >> though. > There is no such need, its just how things are implemented right now. To > allow IOAPIC/PIC to be in userspace and LAPIC in kernel additional > interfaces are needed and since we need to keep current way for > foreseeable feature the value of doing the work is not clear. > >> KVM_INTERRUPT, the docs say, "is only useful if in-kernel local >> APIC or equivalent is not used", but it is really only useful for if >> in-kernel *IOAPIC* is not used. The userspace IOAPIC can use it to >> inject the interrupts to the in-kernel LAPIC. > KVM_INTERRUPT is synchronous. Ah, I think that's where my thinko is. On PPC, KVM_INTERRUPT is asynchronous. Alex > There is no reason for userspace > IOAPIC<->kernel space LAPIC interface to be synchronous. KVM_IRQ_LINE is > much better fit. > >> >> So, it would be possible to create the IOAPIC or PIC separately with >> KVM_CREATE_DEVICE, and have the userspace devices inject the interrupts >> with KVM_IRQ_LINE_STATUS (PIC->IOAPIC) or KVM_INTERRUPT (IOAPIC->LAPIC). >> > PIC never injects into IOAPIC. Some GSIs are handled by both. > > -- > Gleb. -- 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
