Re: [PATCH v2 1/8] powerpc/32: Add VDSO version of getcpu

[Christophe Leroy]

Le 18/09/2019 Ã 07:51, Michael Ellerman a ÃcritÂ:
> We are still in the middle of the years long process of removing the
> "magic" syscall on 64-bit:
>
>    https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/powerpc/kernel/exceptions-64s.S?commit=4d856f72c10ecb060868ed10ff1b1453943fc6c8#n1578
>   
>
> Can we not add another one on 32-bit?
>
> Is it really such a fast path that it's worth putting a wart in the
> syscall entry like that?
>
> Is there some other method? On s390 they have a per-cpu VDSO page, that
> would be a nice option. How we do that would be specific to a particular
> MMU, and maybe not even possible with some MMUs. So maybe that's not
> feasible.

Ok, for now I remove the fast syscall and only keep the VDSO getcpu() 
for non SMP.

I think we may in the future implement a per-cpu VDSO page that will be 
mapped to the process based on the CPU it is running on. I need to look 
at that in more details.

Christophe