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

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Date:	Mon, 11 Jan 2010 16:28:30 -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 04:06 PM, Suresh Siddha wrote:
>>>
>>> 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.
>> 
>
> OK, fair enough.
>
>> 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?
>
> It sounds like something that we could experiment with -- after
> switching an IRQ to ioapic mode, make it a movable interrupt.  It
> *seems* it should work, but it's scary stuff to muck with.
>
> Eric, do you see any reason why it wouldn't work?  I truly couldn't
> understand your previous remark, especially the bit about "it is
> dangerous to play lowest priority irq games in that range".

Sorry.  I suck at multitasking.

Without changes assign_irq_vector will reuse vectors in the range
IRQ0_VECTOR to IRQ15_VECTOR in the code as it we currently ship it,
when we switch irq0-15 into ioapic mode.

Switching the loop to cover IRQ0_VECTOR to IRQ15_VECTOR is not a
problem.  I don't think it will find anything free as we assign those
vectors on all cpus, but the data structures are fine.

I am uncomfortable with the suggestion of sharing the priority of the
IRQ_MOVE_CLEANUP_VECTOR with other interrupts.  I know if it had be
clear from the documentation that it was safe to share the irq level
with other interrupts I would not have reserved the entire interrupt
level for the IRQ_MOVE_CLEANUP_VECTOR.

Eric
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