From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [RFC][PATCH] irq: remove IRQF_DISABLED
Date: Fri, 06 Mar 2009 18:02:24 +0100
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To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>, Ingo Molnar <mingo@elte.hu>, Thomas Gleixner <tglx@linutronix.de>, lkml <linux-kernel@vger.kernel.org>, linux-arch <linux-arch@vger.kernel.org>, Andrew Morton <akpm@linux-foundation.org>

On Fri, 2009-03-06 at 11:24 +0100, Peter Zijlstra wrote:
> On Fri, 2009-03-06 at 20:59 +1100, Benjamin Herrenschmidt wrote:
> > > If you have distinct interrupt priorities, you can
> > > 
> > > 1) provide an interrupt stack for each priority
> > > 2) mask all lower priorities when handling one
> > > 
> > > Would that not work?
> > 
> > The PIC does that already. IE. it will only interrupt again before
> > ->eoi() for an interrupt of a higher priority. But by using
> > IRQF_DISABLED, you mask interrupts in the core, and thus effectively
> > completely prevents the whole thing.
> > 
> > > The problems with enabling irqs in hardirq handlers are that you get
> > > unlimited irq nesting, which is bad for your stack, furthermore, somehow
> > > people thing it makes things 'faster' because the irq-off latency goes
> > > down.
> > 
> > No, you don't get unlimited IRQ nesting, at least not on sane archs with
> > a decent PIC that does things like what I described above :-)
> 
> Right, welcome to x86 ;-)

Ok, people put me straight here. Since linux not support interrupt
priorities, wouldn't it simply be a matter of implementing
local_irq_en/dis-able() as masking the lowest level you use to run
normal interrupts on?

That will leave your other interrupt level available as NMI/debug
thingies.