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

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Message-ID: <4B5C00EB.3000208@zytor.com>
Date:	Sun, 24 Jan 2010 00:12:27 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"Maciej W. Rozycki" <macro@...ux-mips.org>
CC:	Suresh Siddha <suresh.b.siddha@...el.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 01/23/2010 09:52 PM, Maciej W. Rozycki wrote:
> 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?
>

The difference is relevant when they are *not* invoked as ExtInt 
interrupts, but when used as IOAPIC interrupts it matters.

	-hpa
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