Re: [Xenomai-help] FPU not available

[Jan Kiszka <jan.kiszka@domain.hid>]

Jan Kiszka wrote:
> Gilles Chanteperdrix wrote:
>> On Wed, Feb 6, 2008 at 3:09 PM, Petr Cervenka <grugh@domain.hid> wrote:
>>> Hello.
>>>  Recently, we switched to newer distribution of linux (Kubuntu 7.10). During this switch we changed many things (Xenomai 2.4.1, linux kernel 2.6.24, x86_64 architecture, ...).
>>>  No we have problem, that in one of our tasks we are sometimes not able to use floating point operations (under very specific circumstances) . In such case, that task crashes immediately, but rest of the application runs "normaly". Output from dmesg is attached to this message. Task was created with T_FPU flag.
>>>  Is there anything we can check or change?
>>>  Petr Cervenka
>> I do not know if this is related to the issue you are facing, but the
>> first FPU fault of a thread running in primary mode may be handled by
>> Xenomai without switching to secondary mode. So, maybe the fault
>> epilogue implicitely expects Xenomai to have switched the fault to
>> secondary mode and use some secondary mode services such as
>> ipipe_restore_root, whereas the thread never leaved primary mode.
>>
> 
> Good point! That is probably this path (and not the one I starred on):
> 
> __ipipe_handle_exception()
> 	...
> 	if (unlikely(ipipe_trap_notify(vector, regs))) {
> 		local_irq_restore(flags);
> 		return 1;
> 	}
> 
> That needs some more thoughts...

Looking at the whole __ipipe_handle_exception, the problem is related to
 the early, context-independent __ipipe_stall_root(). Can we postpone
this safely after having called any potential high-stage hooks for this
exception, and then only if the callee migrated the thread to the root
domain? Or is there a need to have the root domain stalled across the
post-fault migration?

In the latter case, we would have to fiddle with the stall bits directly
instead of calling local_irq_restore - not just to work around the
BUG_ON, but also to avoid sync'ing root over potentially stalled
non-root domains...

Jan

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