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To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Suresh Siddha <suresh.b.siddha@intel.com>, Ingo Molnar <mingo@elte.hu>,
       Thomas Gleixner <tglx@linutronix.de>, Yinghai Lu <yinghai@kernel.org>,
       "Maciej W. Rozycki" <macro@linux-mips.org>,
       LKML <linux-kernel@vger.kernel.org>
Subject: Re: [patch] x86, apic: use 0x20 for the IRQ_MOVE_CLEANUP_VECTOR instead of 0x1f
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	<4B47E7A9.6090904@zytor.com>
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	<4B4BACCA.2040805@zytor.com>
From: ebiederm@xmission.com (Eric W. Biederman)
Date: Mon, 11 Jan 2010 15:10:07 -0800
In-Reply-To: <4B4BACCA.2040805@zytor.com> (H. Peter Anvin's message of "Mon\, 11 Jan 2010 14\:57\:14 -0800")
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"H. Peter Anvin" <hpa@zytor.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