[RFC/WIP] powerpc: Fix 32-bit handling of MSR_EE on exceptions

[Christophe Leroy]

On 02/05/2019 10:10 AM, Michael Ellerman wrote:
> Christophe Leroy <christophe.leroy at c-s.fr> writes:
>> Le 20/12/2018 à 23:35, Benjamin Herrenschmidt a écrit :
>>>
>>>>>     /*
>>>>>      * MSR_KERNEL is > 0x10000 on 4xx/Book-E since it include MSR_CE.
>>>>> @@ -205,20 +208,46 @@ transfer_to_handler_cont:
>>>>>     	mflr	r9
>>>>>     	lwz	r11,0(r9)		/* virtual address of handler */
>>>>>     	lwz	r9,4(r9)		/* where to go when done */
>>>>> +#if defined(CONFIG_PPC_8xx) && defined(CONFIG_PERF_EVENTS)
>>>>> +	mtspr	SPRN_NRI, r0
>>>>> +#endif
>>>>
>>>> That's not part of your patch, it's already in the tree.
>>>
>>> Yup rebase glitch.
>>>
>>>    .../...
>>>
>>>> I tested it on the 8xx with the below changes in addition. No issue seen
>>>> so far.
>>>
>>> Thanks !
>>>
>>> I'll merge that in.
>>
>> I'm currently working on a refactorisation and simplification of
>> exception and syscall entry on ppc32.
>>
>> I plan to take your patch in my serie as it helps quite a bit. I hope
>> you don't mind. I expect to come out with a series this week.
> 
> Ben's AFK so go ahead and pull it in to your series if that helps you.
>   
>>> The main obscure area is that business with the irqsoff tracer and thus
>>> the need to create stack frames around calls to trace_hardirqs_* ... we
>>> do it in some places and not others, but I've not managed to make it
>>> crash either. I need to get to the bottom of that, and possibly provide
>>> proper macro helpers like ppc64 has to do it.
>>
>> I can't see anything special around this in ppc32 code. As far as I
>> understand, a stack frame is put in place when there is a need to
>> save and restore some volatile registers. At the places where nothing
>> needs to be saved, nothing is done. I think that's the normal way for
>> any function call, isn't it ?
> 
> The concern was that the irqsoff tracer was doing
> __builtin_return_address(1) (or some number > 0) and that crashes if
> there aren't sufficiently many stack frames available.
> 
> See ftrace_return_address.
> 
> Possibly the answer is that we don't have CONFIG_FRAME_POINTER and so we
> get the empty version of that.
> 

Yes indeed, ftrace_return_address(1) is not __builtin_return_address(1) 
but 0ul as CONFIG_FRAME_POINTER is not defined. So the crash can't be 
due to that as it would then crash regardless of whether we set a stack 
frame or not.
And anyway, as far as I understand, if the stack is properly 
initialised, __builtin_return_address(X) returns NULL and don't crash 
when the top of backtrace is reached.

Do you have more details about the said crash ? I think we should file 
an issue for it in our issue databse.

For the time being, I'll get rid of that unneccessary stack frame in 
entry_32.S as part of my syscall prolog optimising series.

Christophe