linux-kernel - Re: [PATCH] x86: mark some mpspec inline functions as _

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Date:   Thu, 25 Feb 2021 23:16:20 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] x86: mark some mpspec inline functions as __init

On Thu, Feb 25, 2021 at 10:59 PM 'Nick Desaulniers' via Clang Built
Linux <clang-built-linux@...glegroups.com> wrote:
> On Thu, Feb 25, 2021 at 1:33 PM Borislav Petkov <bp@...en8.de> wrote:
> >
> > But apparently the cost model has decided that this is not inlineable.
> > Maybe because that function ptr is assigned at boot time and that
> > somehow gets the cost model to give it a very high (or low) value. Or
> > maybe because the wrappers are calling through a variable - the x86_init
> > thing - which is in a different section and that confuses the inliner.
> > Or whatever - totally speculating here.
>
> The config that reproduces it wasn't shared here; I wouldn't be
> surprised if this was found via randconfig that enabled some config
> that led to excessive code bloat somewhere somehow.

It was the case in some similar bugfixes I sent, but in this one, the
out-of-line function was fairly trivial -- there is one call to a sanitizer
but that's it:

        .section        .init.text,"ax",@progbits
        .type   early_get_smp_config,@function  # -- Begin function
early_get_smp_config
early_get_smp_config:                   # @early_get_smp_config
# %bb.0:
        callq   __sanitizer_cov_trace_pc@PLT
        movl    $1, %edi
        jmpq    *x86_init+40(%rip)              # TAILCALL
.Lfunc_end4:
        .size   early_get_smp_config, .Lfunc_end4-early_get_smp_config

I've uploaded the .config to https://pastebin.com/CvvEScnQ

This is with today's linux-next-20210225 plus a number of patches
to address other build failures.

          Arnd