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

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Message-ID: <871so1w86g.fsf@concordia.ellerman.id.au>
Date:   Thu, 24 Aug 2017 20:51:03 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Christophe Leroy <christophe.leroy@....fr>,
        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()

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.

cheers