On 09/17/2012 08:15 PM, Jan Kiszka wrote:
> On 2012-09-17 20:12, Jan Kiszka wrote:
>> On 2012-09-17 20:08, Gilles Chanteperdrix wrote:
>>> On 09/17/2012 08:05 PM, Jan Kiszka wrote:
>>>
>>>> On 2012-09-17 19:46, Gilles Chanteperdrix wrote:
>>>>> ipipe_end is a nop when called from primary domain, yes, but this is not
>>>>> very different from edge irqs. Also, fasteoi become a bit like MSI: in
>>>>> the same way as we can not mask MSI from primary domain, we should not
>>>>> mask IO-APIC fasteoi irqs, because the cost is too prohibitive. If we
>>>>> can live with MSI without masking them in primary mode, I guess we can
>>>>> do the same with fasteoi irqs.
>>>>
>>>> MSIs are edge triggered, fasteois are still level-based. They require
>>>> masking at the point you defer them - what we do and what Linux may even
>>>> extend beyond that. If you mask them by raising the task priority, you
>>>> have to keep it raised until Linux finally handled the IRQ.
>>>
>>>
>>> Yes.
>>>
>>>> Or you
>>>> decide to mask it at IO-APIC level again.
>>>
>>>
>>> We do not want that.
>>>
>>>> If you keep the TPR raised,
>>>> you will block more than what Linux wants to block.
>>>
>>>
>>> The point is that if the TPR keeps raised, it means that primary domain
>>> has preempted Linux, so, we want it to keep that way. Otherwise the TPR
>>> gets lowered when Linux has handled the interrupt.
>>>
>>> A week-end of testing made me sure of one thing: it works. I assure you.
>>
>> Probably, in the absence of IRQF_ONESHOT Linux interrupts. No longer if
>> you face threaded IRQs - I assure you.
>
> Well, it may work (if mask/unmask callbacks work as native) but the
> benefit is gone: masking at IO-APIC level will be done again. Given that
> threaded IRQs become increasingly popular, it will also be hard to avoid
> them in common setups.
The thing is, if we no longer use the IO-APIC spinlock from primary
domain, we may not have to turn it into an ipipe_spinlock, and may be
able to preempt the IO-APIC masking.
--
Gilles.
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