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

Christophe LEROY [off-list ref] writes:

Le 24/08/2017 =C3=A0 12:51, Michael Ellerman a =C3=A9crit=C2=A0:

quoted

Christophe Leroy [off-list ref] writes:
=20

quoted

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'

=20
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=20
(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