linux-kernel - Re: [RFC] killing the NR

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Message-ID: <m1ps8ac4fc.fsf@ebiederm.dsl.xmission.com>
Date:	Fri, 16 Feb 2007 12:01:43 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <ak@...e.de>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Ingo Molnar <mingo@...e.hu>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [RFC] killing the NR_IRQS arrays.

Jeremy Fitzhardinge <jeremy@...p.org> writes:

> Eric W. Biederman wrote:
>> So I propose we remove all assumptions from the code that we actually
>> have an array of irqs.  That will allow for irq_desc to be dynamically
>> allocated instead of statically allocated saving memory and reducing
>> kernel complexity.
>>   
>
> Sounds good to me.  In Xen we have 1024 event channels which we need to
> map down into a smaller irq.  Aside from the complexity of maintaining a
> mapping table, that's not a huge issue for now, but when we start
> exposing pci devices to guests it all becomes more complex.  The ideal
> for us is to simply use event channel == irq, which this would allow.

Well you shouldn't need to wait just run with a kernel with NR_IRQS >= 1024.
1024 is stretch but it isn't to bad.  There are already x86 boxes that have
more pins on their ioapics then that. So x86_64 and with this latest
round of patches from Len Brown and I i386 should be able to support that.

On the other side 1024 looks extremely limiting for exposing pci devices.  
If someone gets serious about pushing what is legal with MSI-X you may be
in trouble.  As a single device is allowed to have 4096 interrupts.   Not
that I can think of a user for so many but...

Eric
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