Re: [patch] x86, apic: use 0x20 for the IRQ_MOVE_CLEANUP_VECTOR instead of 0x1f

["H. Peter Anvin" <hpa@zytor.com>]

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