Philippe Gerum wrote:
> On Thu, 2007-10-04 at 16:06 +0200, Jan Kiszka wrote:
>> Philippe Gerum wrote:
>>> On Thu, 2007-10-04 at 14:42 +0200, Jan Kiszka wrote:
>>>> Philippe Gerum wrote:
>>>>> On Thu, 2007-10-04 at 11:34 +0200, Philippe Gerum wrote: 
>>>>>>> Well, this trace also reveals a second bug that can cause nasty priority
>>>>>>> inversion: a high-prio domains executes when a fasteoi-IRQ arrives for a
>>>>>>> low-prio domain. This will now block all IRQs until the low-prio domain
>>>>>>> was able to run its IRQ handler completely. Thus we must _mask_ fasteoi
>>>>>>> IRQs for low-prio domains while high-prio ones are running!
>>>>>>>
>>>>>> This code was actually there up to 2.6.17-1.5-02, and was removed at
>>>>>> some point in the 2.6.19 series, due to some severe conflicts with the
>>>>>> vanilla IO-APIC support which used to be a hell of a moving target at
>>>>>> that time. I guess it's time to bring this code back.
>>>>>>
>>>>> Does the following work for you?
>>>> Will give it a try later. Meanwhile...
>>>>
>>>>> diff --git a/arch/i386/kernel/io_apic.c b/arch/i386/kernel/io_apic.c
>>>>> index 2ae79e9..517937b 100644
>>>>> --- a/arch/i386/kernel/io_apic.c
>>>>> +++ b/arch/i386/kernel/io_apic.c
>>>>> @@ -2022,6 +2022,8 @@ static void ack_ioapic_quirk_irq(unsigned int irq)
>>>>>           __unmask_and_level_IO_APIC_irq(irq);
>>>>>           spin_unlock(&ioapic_lock);
>>>>>   }
>>>>> +
>>>>> + __mask_IO_APIC_irq(irq);
>>>>>  }
>>>> ...I have problems understanding this hunk. Typo? Should this read
>>>> __unmask_IO_APIC_irq?
>>>>
>>> No, you want to mask it here. EOI in the IO-APIC case goes through some
>>> quirks which you want to apply immediately on behalf of the primary
>>> I-pipe ack handler, basically to work around some IO-APIC errata. Then,
>>> either the high priority domain (__ipipe_end_fasteoi_irq) or the root
>>> one (handle_fasteoi_irq) will unmask the IRQ as needed, whichever comes
>>> first (and only).
>> ack_ioapic_quirk_irq == eio for fasteoi, so it is specifically executed
>> on exit of handle_fasteoi_irq. I still don't see why you want to leave
>> the IRQ masked here.
>>
> 
> It is not executed on IRQ exit anymore when the I-pipe is enabled. The
> EOI handler is called earlier in the latter case to ack the LAPIC, then
> mask the interrupt source from the IO-APIC, waiting for the Linux
> handler to process the device which triggered the interrupt. The source
> is eventually unmasked when either the high priority domain or Linux is
> done with the interrupt.

Ah, ok, too blind to see the full picture: handle_fasteoi_irq was
changed in that direction.

Jan (who just kicked off a patched kernel rebuild)

-- 
Siemens AG, Corporate Technology, CT SE 2
Corporate Competence Center Embedded Linux

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