Re: [PATCH v2] powerpc: Fix irq_soft_mask_set() and irq_soft_mask_return() with sanitizer

[Peter Bergner <bergner@linux.ibm.com>]

On 8/31/22 5:45 PM, Segher Boessenkool wrote:
> On Tue, Aug 30, 2022 at 09:10:02AM +0000, Christophe Leroy wrote:
>> Le 30/08/2022 à 11:01, Nicholas Piggin a écrit :
>>> On Tue Aug 30, 2022 at 3:24 PM AEST, Christophe Leroy wrote:
>>>>> This is still slightly concerning to me. Is there any guarantee that the
>>>>> compiler would not use a different sequence for the address here?
>>>>>
>>>>> Maybe explicit r13 is required.
>>>>>
>>>>
>>>> local_paca is defined as:
>>>>
>>>> 	register struct paca_struct *local_paca asm("r13");
> 
> And this is in global scope, making it a global register variable.
> 
>>>> Why would the compiler use another register ?
>>>
>>> Hopefully it doesn't. Is it guaranteed that it won't?
> 
> Yes, this is guaranteed.

Agree with Segher here.  That said, there was a gcc bug a looooong time
ago where gcc copied r13 into a temporary register and used it from there.
That's ok (correctness wise, but not ideal) from user land standpoint,
but we took a context switch after the reg copy and it was restarted on
a different cpu, so differnt local_paca and r13 value.  We went boom
because the copy wasn't pointing to the correct local_paca anymore.
So it is very important the compiler always use r13 when accessing
the local_paca.

Peter