Re: [PATCH 07/13] irqdomain: Add max_affinity argument to irq_domain_alloc_descs()

[David Woodhouse <dwmw2@infradead.org>]

On 7 October 2020 16:57:36 BST, Thomas Gleixner <tglx@linutronix.de> wrote:
>On Wed, Oct 07 2020 at 15:10, David Woodhouse wrote:
>> On Wed, 2020-10-07 at 15:37 +0200, Thomas Gleixner wrote:
>>> What is preventing you to change the function signature? But handing
>>> down irqdomain here is not cutting it. The right thing to do is to
>>> replace 'struct irq_affinity_desc *affinity' with something more
>>> flexible.
>>
>> Yeah, although I do think I want to ditch this part completely, and
>> treat the "possible" mask for the domain very much more like we do
>> cpu_online_mask. In that we can allow affinities to be *requested*
>> which are outside it, and they can just never be *effective* while
>> those CPUs aren't present and reachable.
>
>Huch?
>
>> That way a lot of the nastiness about preparing an "acceptable" mask
>in
>> advance can go away.
>
>There is not lot's of nastiness.

OK, but I think we do have to cope with the fact that the limit is dynamic, and a CPU might be added which widens the mask. I think that's fundamental and not x86-specific.

>> It's *also* driven, as I noted, by the realisation that on x86, the
>> x86_non_ir_cpumask will only ever contain those CPUs which have been
>> brought online so far and meet the criteria... but won't that be true
>> on *any* system where CPU numbers are virtual and not 1:1 mapped with
>> whatever determines reachability by the IRQ domain? It isn't *such*
>an
>> x86ism, surely?
>
>Yes, but that's exactly the reason why I don't want to have whatever
>mask name you chose to be directly exposed and want it to be part of
>the
>irq domains because then you can express arbitrary per domain limits.

The x86_non_ir_mask isn't intended to be directly exposed to any generic IRQ code.

It's set up by the x86 APIC code so that those x86 IRQ domains which are affected by it, can set it with irqdomain_set_max_affinity() for the generic code to see. Without each having to create the cpumask from scratch for themselves.

> ... reading for once the kids are elsewhere...

Thanks.

>It's not rocket science to fix that as the irq remapping code already
>differentiates between the device types.

I don't actually like that very much. The IRQ remapping code could just compose the MSI messages that it desires without really having to care so much about the child device. The bit where IRQ remapping actually composes IOAPIC RTEs makes me particularly sad.

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