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

[Michael Ellerman]

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.

cheers