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

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Message-ID: <4fcbdc51-b170-d4a2-5344-c55b2ec71cbb@c-s.fr>
Date:   Thu, 24 Aug 2017 15:58:41 +0200
From:   Christophe LEROY <christophe.leroy@....fr>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Scott Wood <oss@...error.net>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 4/4] powerpc/32: remove a NOP from memset()



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