linux-kernel - Re: [tip:x86/irq] x86: apic: Fix mismerge, add arch_probe_nr

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Date:	Mon, 01 Mar 2010 13:58:55 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Ian Campbell <ijc@...lion.org.uk>
Cc:	Yinghai Lu <yinghai@...nel.org>, mingo@...hat.com, hpa@...or.com,
	linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...e.hu,
	linux-tip-commits@...r.kernel.org,
	Jeremy Fitzhardinge <jeremy@...p.org>
Subject: Re: [tip:x86/irq] x86: apic: Fix mismerge, add arch_probe_nr_irqs() again

Ian Campbell <ijc@...lion.org.uk> writes:

> On Mon, 2010-03-01 at 10:34 -0800, Eric W. Biederman wrote:
>> 
>> As of 2.6.33 the limitations in DomU support are:
>> - xen_evtchn_do_upcall starts with the irq number instead of
>>   the irq_desc, and happens to unnecessarily call into arch
>>   specific code.  
>
> I saw a patch to fix this one recently, "xen: Remove unnecessary arch
> specific xen irq functions.", right?

Yes.  

You probably want to modify evtnchn_to_irq to return an irq_desc.

It is going to take a bit but our next big step for the irq methods
is to make them all take struct irq_desc pointers instead of unsigned
int irq, so we don't have to repeat the lookups.

>> - Xen has an array irq_info[NR_IRQS] one of the last static arrays
>>   sized at NR_IRQs in the entire kernel.
>
> Hopefully the same info as is in that array could (and indeed should) be
> instead stored in irq_desc->chip_data. Would you object to
> arch_init_copy_chip_data and arch_free_chip_data becoming function
> pointers within the struct irq_chip?

No objections.  Now that I see those methods it looks like they always
should have been in irq_chip.

>> If you can fix the Xen code so it isn't dragging the rest of the
>> kernel down when it comes to large numbers of irqs more power to you.
>
> If you know of other aspects of the Xen code where this is the case (or
> find them in the future) please let me know, I'll do my best to fix
> them.

Good to hear.

Right now I am being a bit of a jack in the box reviewer.  I don't have
time to work on the irq code right now but I occasionally pop out of
my box and review the code and try to keep it from descending into an
unmaintainable disaster.

Eric
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