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Date: Fri, 20 Feb 2015 20:44:56 +0100
From: Philippe Gerum <rpm@xenomai.org>
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Subject: Re: [Xenomai] ipipe: issues with ARM exception handling
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To: Jan Kiszka <jan.kiszka@siemens.com>, Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>, Xenomai <xenomai@xenomai.org>

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.

-- 
Philippe.