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SubjectRe: Question about interrupt routing and irq allocation
Ingo Molnar wrote:
> hm, in theory the highest quality method would be to do this on the
> genirq level and register your own special "Xen irq-chip" methods. [see
> include/linux/irq.h's "struct irq_chip" and kernel/irq/*.c]
>

I already have one of those for pv guests, and I think I can reuse it
more or less unchanged.

> you can use set_irq_chip() to claim a specific irq and set up its
> handling at the highest level. That way you dont have to do anything in
> the x86 hw vector space at all and you'd avoid all the overhead and
> complications of x86 irq vectors. You can control how these interrupts
> are named in /proc/interrupts, etc.
>

Yeah, that was my plan.

> but this needs synchronization with all the other entities that claim
> specific irqs and expect to be able to get them. MSI already does that
> to a certain level, see arch_setup_msi_irq() / set_irq_msi(). But that
> wastes x86 vectors and we dont really want to waste them as you dont
> actually want to use any separate per irq hw vectoring mechanism for
> these interrupts.
>

OK. So if I just used create_irq() that would get me an irq I can use,
but would also end up allocating a vector too.

> So the most intelligent method would be to reserve the Linux irq itself
> but not the vector, i.e. allocate from irq_cfg[] in
> arch/x86/kernel/io_apic_64.c so that the irq number does not get reused
> - setting irq_cfg[irq].vector to -1 will achieve that.
>

I'm initially targeting 32-bit, though obviously I'd like something that
works for both 32 and 64 bit. irq_cfg[] is missing in io_apic_32.c;
would I achieve the same effect by setting irq_vector[irq] = 0xff or
something?

Thanks,
J


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