On Tuesday 02 May 2006 08:57, Eric W. Biederman wrote:
> Andi Kleen <[email protected]> writes:
>
> >> >- Modify do_IRQ to get passed an interrupt vector# from the
> >> > interrupt vector instead of an irq number, and then lookup
> >> > the irq number in vector_irq. This means we don't need
> >> > a code stub per irq, and allows us to handle more irqs
> >> > by simply increasing NR_IRQS.
> >>
> >> isn't the vector number already on the stack from
> >> ENTRY(interrupt)
> >> pushl $vector-256
> >
> > Yes - and interrupts/vectors are currently always identical.
>
> No. At best there is a fixed offset. They can't be
> identical because the first 32 vectors are reserved,
> for processor exceptions.
>
> Beyond that the kernel would not need the vector_irq and irq_vector
> arrays if they were always identical, or even if they were one to one.
Yes I should have said it's a fixed offset. Sorry for the confusion.
Just no mapping table needed.
>
> If you look at assign_irq_vector you will see that by default we
> allocate every 8th vector. Looking at the comment in
> init_IO_APIC_traps() this seems to be because we want to avoid
> apic bugs with multiple interrupts of the same priority.
> Although why we skip 8 instead of 16 is beyond me.
Hmm - i guess that's old APIC bugs. Might be worth revisiting
on 64bit.
>
> Although now that I think about it, using some assembler macros
> instead of cpp macros could probably solve the problem more easily
> than generating the stubs at runtime. I think the worst case is
> 256 cpus * 32 irqs per cpu
32 irqs? It's (255-32)
> * 10 bytes per stub = 80K.
My calculations gave >200k which is definitely too much.
-Andi
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