linux-kernel - Re: [PATCH 00/46] gcc-LTO support for the kernel

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Date:   Thu, 17 Nov 2022 09:28:00 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     "Jiri Slaby (SUSE)" <jirislaby@...nel.org>,
        linux-kernel@...r.kernel.org,
        Alexander Potapenko <glider@...gle.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Alexey Makhalov <amakhalov@...are.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Ben Segall <bsegall@...gle.com>,
        Borislav Petkov <bp@...en8.de>,
        Daniel Borkmann <daniel@...earbox.net>,
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        Yonghong Song <yhs@...com>
Subject: Re: [PATCH 00/46] gcc-LTO support for the kernel

On Mon, Nov 14, 2022 at 08:40:50PM +0100, Ard Biesheuvel wrote:
> On Mon, 14 Nov 2022 at 12:44, Jiri Slaby (SUSE) <jirislaby@...nel.org> wrote:
> >
> > Hi,
> >
> > this is the first call for comments (and kbuild complaints) for this
> > support of gcc (full) LTO in the kernel. Most of the patches come from
> > Andi. Me and Martin rebased them to new kernels and fixed the to-use
> > known issues. Also I updated most of the commit logs and reordered the
> > patches to groups of patches with similar intent.
> >
> > The very first patch comes from Alexander and is pending on some x86
> > queue already (I believe). I am attaching it only for completeness.
> > Without that, the kernel does not boot (LTO reorders a lot).
> >
> > In our measurements, the performance differences are negligible.
> >
> > The kernel is bigger with gcc LTO due to more inlining.
> 
> OK, so if I understand this correctly:
> - the performance is the same
> - the resulting image is bigger
> - we need a whole lot of ugly hacks to placate the linker.
> 
> Pardon my cynicism, but this cover letter does not mention any
> advantages of LTO, so what is the point of all of this?

Seconded; I really hate all the ugly required for the GCC-LTO
'solution'. There not actually being any benefit just makes it a very
simple decision to drop all these patches on the floor.