Re: Subject: [PATCH 1/2] x86: get back 15 vectors

[ebiederm@xmission.com (Eric W. Biederman)]

Yinghai Lu <yinghai@kernel.org> writes:

> On 01/04/2010 11:09 AM, Eric W. Biederman wrote:
>> "H. Peter Anvin" <hpa@zytor.com> writes:
>> 
>>> On 01/04/2010 08:18 AM, Eric W. Biederman wrote:
>>>> Yinghai Lu <yinghai@kernel.org> writes:
>>>>
>>>> This patch is wrong.
>>>>
>>>>> between FIRST_EXTERNAL_VECTOR (0x20) and FIRST_DEVICE_VECTOR (0x41)
>>>>>
>>>>> for 0x20 and 0x2f, we are safe be used_vectors will prevent it to use used one.
>>>>
>>>> We can not use any of 0x20 - 0x2f for ioapic irqs.  We need the entire
>>>> priority level to ensure that the irq move cleanup ipi is of a lower
>>>> priority.
>>>>
>>>
>>> Almost makes one want to abuse 0x1f for that.  Although 0x00..0x1f are
>>> reserved for exceptions, the APICs range down to 0x10, and well, when
>>> 0x1f ends up actually getting used as an exception vector that we
>>> support, then we can trivially change that.  In the meantime it would
>>> actually make use of an otherwise-unusable APIC priority level.
>> 
>> An optimization like that (with a big fat comment) seems reasonable
>> to me.
>
> so we can use [0x10, 0x1f]
>
> sth like this?

Something.  We can not use all of 0x10 - 0x1f, it is simply
that hardware can address all of that. 0x10 is already defined
as something I forget what.   0x12 is already the MCE_VECTOR.


Since hardware has not yet defined 0x1f (and is not likely to for
a while.  We can use that).  So we wind up using hardware priority
a single ipi, and hardware exceptions.

Eric