Re: [Xenomai-core] [PATCH v2] x86: Fix root domain state restoring on exception return

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

Wolfgang Mauerer wrote:
> Jan Kiszka wrote:
>> Wolfgang Mauerer wrote:
>>> Jan Kiszka wrote:
>>>> Wolfgang Mauerer wrote:
>>>>> Jan Kiszka wrote:
>>>>>> Wolfgang Mauerer wrote:
>>>>>>> Jan Kiszka wrote:
>>>>>>>> If we enter __ipipe_handle_exception over a non-root domain and leave it
>>>>>>>> due to migration in the event handler over root, we must not restore the
>>>>>>>> root domain state so far saved on entry. This caused subtle pipeline
>>>>>>>> state corruptions. Instead, only save and restore them if we were
>>>>>>>> entering over root.
>>>>>>>>
>>>>>>>> However, the x86-32 regs.flags fixup is required nevertheless to take
>>>>>>>> care of mismatches between the root domain state and the hardware flags
>>>>>>>> on entry. That may happen if we fault in the iret path. But also in this
>>>>>>>> case we must not restore an invalid root domain state. So if we entered
>>>>>>>> over non-root, pick up the input for __fixup_if from the root domain
>>>>>>>> after running the ipipe handler.
>>>>>>>>
>>>>>>>> Signed-off-by: Jan Kiszka <jan.kiszka@domain.hid>
>>>>>>>> ---
>>>>>>>>
>>>>>>>> Next try. But this time I think I finally understood what scenario
>>>>>>>> __fixup_if is actually fixing. Please correct me if I'm still missing
>>>>>>>> one.
>>>>>>> looks good - it works for my test cases and solves the problems with
>>>>>>> the hw/pipeline state mismatch during early bootup. But do you happen
>>>>>>> to have any scenario at hand with ipipe_domain_root_p && !root_entry?
>>>>>>> Couldn't trigger this one yet so only the raw_irqs_disabled_flags
>>>>>>> fixup is excercised, though I guess it can't do any harm to really
>>>>>>> ensure that the explanation fits reality this time...
>>>>>> You mean non-root entry -> migration -> __fixup_if? In that case we pick
>>>>>> up the flags for fixup _after_ the migration (raw_irqs_disabled()). Or
>>>>>> what do you mean?
>>>>> (...)
>>>>>>>> -	if (unlikely(!ipipe_root_domain_p)) {
>>>>>>>> +	if (likely(ipipe_root_domain_p)) {
>>>>>>>> +		/*
>>>>>>>> +		 * 32-bit: In case we faulted in the iret path, regs.flags do
>>>>>>>> +		 * not match the root domain state as the low-level return
>>>>>>>> +		 * code will evaluate it. Fix this up, either by the root
>>>>>>>> +		 * state sampled on entry or, if we migrated to root, with the
>>>>>>>> +		 * current state.
>>>>>>>> +		 */
>>>>>>>> +		__fixup_if(root_entry ? raw_irqs_disabled_flags(flags) :
>>>>>>>> +					raw_irqs_disabled(), regs);
>>>>> I'm referring to the case that evaluates to
>>>>> __fixup_if(raw_irqs_disabled(), regs); That is, something that
>>>>> triggers
>>>>>
>>>>> 		if (!root_entry)
>>>>> 			do_something();
>>>>>
>>>>> Could be that we're talking about to the same case, although I'm not
>>>>> sure ;-)
>>>> Right, that's the case I described above. What problem do you precisely
>>>> see or what concerns to you have about the suggested behavior?
>>> None. I'd just like to be able to trigger it to avoid that
>>> there are any unforseen problems we're still missing.
>>> Since this corner of Ipipe seems to have proven tricky before
>>> AFAIK, I thought it might perhaps be worth while to really excercise
>>> every possible code path.
>> To trigger it, you need to recreate the scenario that our colleagues
>> managed to generate on the MARS: Have Linux IRQs disabled (or enabled,
>> to have both variations), schedule in a Xenomai task, let it raise a
>> fault so that Xenomai migrates it back.
> 
> Yep, but allocating a large amount of memory in non-rt, switching to
> rt and then accessing this and switching back to non-rt
> in the hope of generating many faults so that the scenario would be
> synthesised without expensive and hard to access hardware was
> unsuccessful, so my little innocent question was really just if you
> happened to have a testcase.

OK, then I totally misunderstood your question.

You do not need to generate a fixable fault, and unfixable one (*(int
*)NULL = 0;) should suffice.

Jan

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