linux-kernel - Re: [patch] x86, apic: use 0x20 for the IRQ_MOVE_CLEANUP

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Date:	Mon, 11 Jan 2010 15:10:07 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Suresh Siddha <suresh.b.siddha@...el.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Yinghai Lu <yinghai@...nel.org>,
	"Maciej W. Rozycki" <macro@...ux-mips.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [patch] x86, apic: use 0x20 for the IRQ_MOVE_CLEANUP_VECTOR instead of 0x1f

"H. Peter Anvin" <hpa@...or.com> writes:

> On 01/11/2010 02:53 PM, Suresh Siddha wrote:
>> 
>>> However, my most serious concern with this patch is that there is a
>>> fairly significant change due to this patch, which is that the legacy
>>> IRQ vectors now fall *inside* the FIRST_DEVICE_VECTOR range.  This isn't
>>> a bad thing -- in fact, it is fundamentally the right thing to do
>>> especially once we consider platforms which *don't* have the legacy IRQs
>>> -- but it makes me scared of unexpected behavior changes as a result.
>>> If you feel confident that that is not the case, could you outline why
>>> it shouldn't be a problem?
>> 
>> In irqinit.c, we statically pre-assign the per-cpu vector to irq
>> mappings (vector_irq) for all the legacy IRQ vectors. Similarly irq_cfg
>> is statically initialized for legacy IRQ's in io_apic.c. So we won't be
>> able to use this space for anything else.
>> 
>
> What enforces that, though?  The used_vector bitmap?  In the past it was
> enforced simply by being < FIRST_DEVICE_VECTOR.

I believe historically it was simply that we did not loop over that set of vectors,
in assign_irq_vector.

Eric
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