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

[Michael Ellerman <mpe@ellerman.id.au>]

Christophe LEROY <christophe.leroy@c-s.fr> writes:

> Le 24/08/2017 à 12:51, Michael Ellerman a écrit :
>> Christophe Leroy <christophe.leroy@c-s.fr> 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),

Haha, you expect me to remember things I said then! ;)

> the ALT_FTR does things too early, while the cache is not enabled yet.

OK. Ben did do some reworks to the early init since then, but I don't
think he changed that.

I notice we do setup_feature_keys() in machine_init(), which is the jump
label equivalent of apply_feature_fixups(). So I wonder if we could
actually move apply_feature_fixups() to there. But it would need some
serious review.

cheers