Re: [patch 2/2] x86, irq: use 0x20 for the IRQ_MOVE_CLEANUP_VECTORinstead of 0x1f

From: H. Peter Anvin
Date: Sun Jan 24 2010 - 03:13:27 EST

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

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

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