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

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

Christophe

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[PATCH 4/4] powerpc/32: remove a NOP from memset() Christophe Leroy <christophe.leroy@c-s.fr> - 2017-08-23 17:00 +0200
  Re: [PATCH 4/4] powerpc/32: remove a NOP from memset() Michael Ellerman <mpe@ellerman.id.au> - 2017-08-24 13:00 +0200
    Re: [PATCH 4/4] powerpc/32: remove a NOP from memset() Christophe LEROY <christophe.leroy@c-s.fr> - 2017-08-24 16:00 +0200
      Re: [PATCH 4/4] powerpc/32: remove a NOP from memset() Michael Ellerman <mpe@ellerman.id.au> - 2017-08-25 02:20 +0200

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