Re: in-kernel interrupt controller steering

[Paolo Bonzini <pbonzini@redhat.com>]

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.

However, it need not mandate the usage of an in-kernel IOAPIC or PIC
though.  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.

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).

Paolo