powerpc: Incorrect stw operand modifier in __set_pte_at

[Christophe Leroy]

Le 08/07/2020 à 16:45, Mathieu Desnoyers a écrit :
> Hi,
> 
> Reviewing use of the patterns "Un%Xn" with lwz and stw instructions
> (where n should be the operand number) within the Linux kernel led
> me to spot those 2 weird cases:
> 
> arch/powerpc/include/asm/nohash/pgtable.h:__set_pte_at()
> 
>                  __asm__ __volatile__("\
>                          stw%U0%X0 %2,%0\n\
>                          eieio\n\
>                          stw%U0%X0 %L2,%1"
>                  : "=m" (*ptep), "=m" (*((unsigned char *)ptep+4))
>                  : "r" (pte) : "memory");
> 
> I would have expected the stw to be:
> 
>                          stw%U1%X1 %L2,%1"
> 
> and:
> arch/powerpc/include/asm/book3s/32/pgtable.h:__set_pte_at()
> 
>          __asm__ __volatile__("\
>                  stw%U0%X0 %2,%0\n\
>                  eieio\n\
>                  stw%U0%X0 %L2,%1"
>          : "=m" (*ptep), "=m" (*((unsigned char *)ptep+4))
>          : "r" (pte) : "memory");
> 
> where I would have expected:
> 
>                  stw%U1%X1 %L2,%1"
> 
> Is it a bug or am I missing something ?

Well spotted. I guess it's definitly a bug.

Introduced 12 years ago by commit 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=9bf2b5cd 
("powerpc: Fixes for CONFIG_PTE_64BIT for SMP support").

It's gone unnoticed until now it seems.

Can you submit a patch for it ?

Christophe