Re: in-kernel interrupt controller steering

[Paolo Bonzini <pbonzini@redhat.com>]

Il 05/03/2013 16:25, Gleb Natapov ha scritto:
>> 1) We need to set the generic interrupt type of the system before we create vcpus.
>>
>> This is a new ioctl that sets the overall system interrupt controller type to a specific model. This used so that when we create vcpus, we can create the appended "local interrupt controller" state without the actual interrupt controller device available yet. It is also used later to switch between interrupt controller implementations.
>>
>> This interrupt type is write once and frozen after the first vcpu got created.
>
> Why explicit ioctl is needed? Why not require specific irqchip to be
> created before first vcpu. The device created determines system interrupt
> controller type.

QEMU creates CPUs before devices, and CPUs need to know what kind of
local interrupt controller to create.  Similar to how in-kernel LAPIC
state is created long before the userspace device that proxies the LAPIC.

I think the above design makes sense.  The alternative would be to
entirely separate the creation of CPUs and devices.  You could even
support heterogeneous systems with some in-kernel irqchips and some
userspace irqchips; sounds cool, but useless too.

>> 7) Interrupt line connections to vcpus are implicit
>>
>> We don't explicitly mark which in-kernel irqchip interrupt line
>> goes to which vcpu. This is done implicitly. If we see a need for it, we
>> create a new irqchip device type that allows us to explicitly configure
>> vcpu connections.
> 
> OK.

Paolo