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

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Message-Id: <1263254812.2859.890.camel@sbs-t61.sc.intel.com>
Date:	Mon, 11 Jan 2010 16:06:47 -0800
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.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

On Mon, 2010-01-11 at 15:13 -0800, H. Peter Anvin wrote:
> On 01/11/2010 03:10 PM, Eric W. Biederman wrote:
> > "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.
> > 
> 
> Yes, that's what I said.  My question was to Suresh what enforces that
> in the case of his patch, which moves the legacy range into the middle
> of the device vectors.

It's not the used_vector bitmap. That range will appear as used on all
the cpu's and hence we won't be allocating it for anything else.

Now the question is: for non-legacy (io-apic) case, instead of reserving
this range for all the cpu's, does it make sense to generalize like any
other vector?

thanks,
suresh

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