linux-kernel - RE: [PATCH] powerpc/32: Move entry_32 functions just after HEAD functions.

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Date:   Thu, 20 Apr 2017 21:25:32 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     David Laight <David.Laight@...LAB.COM>,
        'Christophe Leroy' <christophe.leroy@....fr>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Scott Wood <oss@...error.net>
Cc:     "linuxppc-dev\@lists.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] powerpc/32: Move entry_32 functions just after HEAD functions.

David Laight <David.Laight@...LAB.COM> writes:

> From: Christophe Leroy
>> By default, PPC8xx PINs an ITLB on the first 8M of memory in order
>> to avoid any ITLB miss on kernel code.
>> However, with some debug functions like DEBUG_PAGEALLOC and
>> (soon to come) DEBUG_RODATA, the PINned TLB is invalidated soon
>> after startup so ITLB missed start to happen also on the kernel code.
>> 
>> In order to avoid any ITLB miss in a critical section, we have to
>> ensure that their is no page boundary crossed between the setup of
>> a new value in SRR0/SRR1 and the associated RFI. This cannot be done
>> easily if entry_32 functions sits in the middle of other .text
>> functions. By placing entry_32 just after the .head section (as already
>> done for entry_64 on PPC64), we can more easily ensure the issue
>> doesn't happen.
>
> Shouldn't this be done by putting all the functions that 'matter'
> into a named section instead of relying on the order of the input files?
> (Which is what I think this is doing.)

Yeah that is fragile if there's nothing more to it.

I'm not sure if we need a special section. If the functions that must
not cross a page boundary are aligned to a page boundary (with .align)
then that would also work wouldn't it?

cheers