Re: [Xen-devel] [PATCH] x86: eliminate hard-coded NR_IRQS

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 20/05/2009 06:16, "Jan Beulich" <JBeulich@xxxxxxxxxx> wrote:

> ... splitting it into global nr_irqs (determined at boot time) and per-
> domain nr_pirqs (derived from nr_irqs and a possibly command line
> specified value, which probably should later become a per-domain config
> setting).
> 
> This has the (desirable imo) side effect of reducing the size of struct
> hvm_irq_dpci from requiring an order-3 page to order-2 (on x86-64),
> which nevertheless still is too large.

Erm, well I'm not sure about this patch. Your single stated motivation, to
reduce some struct sizes, could also be addressed by replacing arrays with
other really simple alternatives like hash tables or radix trees. Or
replacing in-place arrays with pointers to arrays (which obviously you do in
some places out of necessity in your patch, but that could equally be done
without making nr_irqs/nr_pirqs dynamic).

Does it make sense to have nr_pirqs > nr_irqs? Are you thinking of a shared
irq being exposed to a guest as non-shared multiple pirqs? Basically I
thought the setting of nr_pirqs based on nr_irqs plus some command-line
values looks a bit bizarre and kludgy and I'm not sure what the usage
scenario is there.

 -- Keir



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