linux-kernel - Re: Xen and incorporating event channels in to nr

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Date:	Wed, 16 Feb 2011 16:12:19 +0000
From:	Ian Campbell <Ian.Campbell@...citrix.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	xen-devel <xen-devel@...ts.xensource.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"Ingo Molnar" <mingo@...e.hu>
Subject: Re: Xen and incorporating event channels in to nr_irqs

On Wed, 2011-02-16 at 15:56 +0000, Thomas Gleixner wrote:
> I'm about to remove the nr_irqs NR_IRQS limitation. It's silly when we
> deal with sparse irqs. So the idea is to have the initial nr_irqs set
> in early boot to have a sensible size for allocating stuff. Later on
> we can expand nr_irqs when the need arises.

> It's not only Xen which wants to eliminate the limitation. Think about
> irq expanders which are detected late in the boot. We have no sensible
> way to reserve enough numbers for them at early boot as we dont know
> whether that hardware is there or not.
> 
> So my plan for .39 is to ignore the NR_IRQS limitation in the sparse
> case and make nr_irqs expandable of course with a sensible upper limit
> in the core code itself. It's basically the allocation bitmap which
> limits it, but I doubt we'll hit 1 Million irq numbers in the
> forseeable future.

That sounds ideal, thanks! 

I was hoping to get rid of the workaround in Xen events.c in the 2.6.39
timeframe too.

If you let me know when you have something I can test I'll combine with
the Xen side and give it a spin.

On a vaguely related note, what is the future of non-sparse IRQs (on x86
and/or generally)?

Ian.

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