linux-kernel - Re: [patch 2/2] x86, irq: use 0x20 for the IRQ_MOVE_CLEANUP

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Date:	Sun, 24 Jan 2010 05:52:01 +0000 (GMT)
From:	"Maciej W. Rozycki" <macro@...ux-mips.org>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
cc:	hpa@...or.com, ebiederm@...ssion.com, yinghai@...nel.org,
	mingo@...e.hu, linux-kernel@...r.kernel.org
Subject: Re: [patch 2/2] x86, irq: use 0x20 for the IRQ_MOVE_CLEANUP_VECTOR
 instead of 0x1f

On Wed, 13 Jan 2010, Suresh Siddha wrote:

> After talking to some more folks inside intel (Peter Anvin, Asit Mallick),
> the safest option (for future compatibility etc) seen was to use vector 0x20
> for IRQ_MOVE_CLEANUP_VECTOR instead of using vector 0x1f (which is documented as
> reserved vector in the Intel IA32 manuals).
> 
> Also we don't need to reserve the entire privilege level (all 16 vectors in
> the priority bucket that IRQ_MOVE_CLEANUP_VECTOR falls into), as the
> x86 architecture (section 10.9.3 in SDM Vol3a) specifies that with in the
> priority level, the higher the vector number the higher the priority.
> And hence we don't need to reserve the complete priority level 0x20-0x2f for
> the IRQ migration cleanup logic.
> 
> So change the IRQ_MOVE_CLEANUP_VECTOR to 0x20 and  allow 0x21-0x2f to be used
> for device interrupts. 0x30-0x3f will be used for ISA interrupts (these
> also can be migrated in the context of IOAPIC and hence need to be at a higher
> priority level than IRQ_MOVE_CLEANUP_VECTOR).

 I have troubles understanding what exactly this change is needed for 
(i.e. what's the difference between using vectors 0x20-0x2f and 0x30-0x3f 
as ExtINT interrupts, what's the gain from relocating them? -- they are 
transparent to the APIC, so the exact priority level used does not matter 
at all), but since I've been cc-ed, I have one question -- have you 
verified that with the new arrangement the mixed interrupt mode (where 
some interrupts come via the APIC and some via the 8259A PICs) still 
works?

  Maciej
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