Re: [Xenomai] ipipe: issues with ARM exception handling

[Philippe Gerum <rpm@xenomai.org>]

On 02/20/2015 08:47 PM, Philippe Gerum wrote:
> On 02/20/2015 08:44 PM, Philippe Gerum wrote:
>> On 02/20/2015 07:17 PM, Jan Kiszka wrote:
>>> On 2015-02-20 19:03, Jan Kiszka wrote:
>>>> Hi Gilles,
>>>>
>>>> analyzing a lockdep warning on 3.16 with I-pipe enabled, I dug deeper
>>>> into the hard and virtual interrupt state management during exception
>>>> handling on ARM. I think there are several issues:
>>>>
>>>> - ipipe_fault_entry should not fiddle with the root irq state if run
>>>>   over head, only when invoked over root.
>>>> - ipipe_fault_exit must not change the root state unless we entered over
>>>>   head and are about to leave over root - see x86. The current code may
>>>>   keep root incorrectly stalled after an exception, though this will
>>>>   probably be fixed up again in practice quickly.
>>>
>>> And the adjustment of the root irq state after migration has to happen
>>> before Linux starts to handle the event. It would basically be a late
>>> ipipe_fault_entry.
>>>
>>>> - do_sect_fault is only called by do_DataAbort and do_PrefetchAbort,
>>>>   in both cases already wrapped in ipipe_fault_entry/exit, thus it
>>>>   shouldn't invoke them once again.
>>>
>>> Sorry, this was a misinterpretation - do_sect_fault is invoked before
>>> ipipe_fault_entry.
>>>
>>> What I need to add, though:
>>>
>>> - do_DataAbort and do_PrefetchAbort call __ipipe_report_trap after
>>>   ipipe_fault_entry, thus with hard IRQs on.
>>
>> This would break LPAE with the Xenomai nucleus as a module on 2.6.x, by
>> treading over a non-linear kernel mapping before the page table could be
>> fixed up. do_translation_fault() must run via the fsr handler
>> indirection before any non-linear access.
>>
> 
> Sorry, if you do that _after_ the fault entry notification, then it's ok
> in theory. However, I don't understand why we would need to notify when
> only a minor fixup is required, that does not entail a mode migration.
> 

To be clearer, do you intend to report the minor fault upon
do_translation_fault() returning zero, or are you referring to a
different context?

-- 
Philippe.