Re: [PATCH 4/4] powerpc/32: remove a NOP from memset()

[Christophe LEROY]

Le 24/08/2017 Ã 12:51, Michael Ellerman a ÃcritÂ:
> Christophe Leroy <christophe.leroy@xxxxxx> writes:
>
> > memset() is patched after initialisation to activate the
> > optimised part which uses cache instructions.
> >
> > Today we have a 'b 2f' to skip the optimised patch, which then gets
> > replaced by a NOP, implying a useless cycle consumption.
> > As we have a 'bne 2f' just before, we could use that instruction
> > for the live patching, hence removing the need to have a
> > dedicated 'b 2f' to be replaced by a NOP.
> >
> > This patch changes the 'bne 2f' by a 'b 2f'. During init, that
> > 'b 2f' is then replaced by 'bne 2f'
>
> I'm not sure what the sequence is during boot for the 32-bit code, but
> can you use an ALT_FTR section for this? Possibly that doesn't get done
> at the right time though.

Unfortunately, as we discussed in 2015 
(https://lkml.org/lkml/2015/9/10/608), the ALT_FTR does things too 
early, while the cache is not enabled yet.

Christophe