Re: [kvmarm] [RFC PATCH 0/3] KVM: ARM: Get rid of hardcoded VGIC addresses

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On 26 October 2012 23:03, Benjamin Herrenschmidt
<benh@xxxxxxxxxxxxxxxxxxx> wrote:
> So the GSI bit. We can assume that GSIs in that context are basically
> our "global interrupt number". This would apply to pretty much every
> platform indeed.
>
> The routing here, if I understand things correctly, consists of
> associating such a global interrupt number with a specific input pin (or
> virtual pin) of a specific source controller (ie, IO APIC).
>
> This would generally make sense in embedded space as well I suppose,
> where you can have multiple or even cascaded interrupt controllers of
> different breeds etc...
>
> However, in the pseries system, this routing is essentially encoded in
> the interrupt number itself. As I think I explained earlier, the number
> is arbitrarily split in two parts, the top bits indicating the source
> controller and the bottom bits the source within that controller. In
> qemu/kvm we have made an arbitrary split (whose size I don't remember
> precisely) and we currently create only one fairly big source controller
> but we might change that in the future.

This is more or less how ARM has done it (though our specific encoding
of interrupt numbers is different, obviously).

If I were designing an interface for this kind of thing from scratch
I'd probably want it to look like "create a specific irqchip and give
me some kind of handle to it" and then have an interface for "assert
interrupt line X on that irqchip". Lacking that, a plausible encoding
scheme on the global interrupt numbers works OK given that you know
there aren't going to be that many irqchips in practice...

-- PMM
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