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Date: Mon, 11 Jan 2010 16:36:17 -0800
From: "H. Peter Anvin" <hpa@zytor.com>
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To: "Eric W. Biederman" <ebiederm@xmission.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
References: <1263002989.2879.664.camel@sbs-t61.sc.intel.com>	<4B47E7A9.6090904@zytor.com>	<1263250418.2859.681.camel@sbs-t61.sc.intel.com>	<4B4BACCA.2040805@zytor.com> <m1tyus8chc.fsf@fess.ebiederm.org>	<4B4BB0B7.3000106@zytor.com>	<1263254812.2859.890.camel@sbs-t61.sc.intel.com>	<4B4BBEBA.4060403@zytor.com> <m17hro6ua9.fsf@fess.ebiederm.org>
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On 01/11/2010 04:28 PM, Eric W. Biederman wrote:
> 
> 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.
> 

What are the properties that you're looking for, and in what
documentation?  We reviewed the Software Development Manual here at
Intel, and it rather explicitly states:

"Each interrupt priority level (sometimes interpreted by software as an
interrupt priority class) encompasses 16 vectors. Prioritizing
interrupts within a priority level is determined by the vector number.
The higher the vector number, the higher the priority within that
priority level.  In determining the priority of a vector and ranking
of vectors within a priority group, the vector number is often divided
into two parts, with the high 4 bits of the vector indicating its
priority and the low 4 bit indicating its ranking within the priority
group."

[Intel® 64 and IA-32 Architectures Software Developer’s Manual
Volume 3A: System Programming Guide, Part 1; September 2009, Order
Number 253668-032US; section 10.9.3, page 10-57f.]

So 0x20 is the lowest-priority vector within priority group 0x2.

	-hpa