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Reply-To: sedat.dilek@gmail.com
From:   Sedat Dilek <sedat.dilek@gmail.com>
Date:   Wed, 10 Mar 2021 22:57:23 +0100
Message-ID: <CA+icZUW8SqpTuANz7brtR+QSxKn+UnYOVve-C+TOMh9RqdQh+w@mail.gmail.com>
Subject: Re: [PATCH] [RFC] arm64: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION
To:     Nicolas Pitre <nico@fluxnic.net>
Cc:     Arnd Bergmann <arnd@kernel.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Nicholas Piggin <npiggin@gmail.com>,
        Fangrui Song <maskray@google.com>,
        Ard Biesheuvel <ardb@kernel.org>,
        Andrew Scull <ascull@google.com>,
        Mark Brown <broonie@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        David Brazdil <dbrazdil@google.com>,
        Geert Uytterhoeven <geert+renesas@glider.be>,
        Ionela Voinescu <ionela.voinescu@arm.com>,
        Kees Cook <keescook@chromium.org>,
        Kristina Martsenko <kristina.martsenko@arm.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Marc Zyngier <maz@kernel.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Will Deacon <will@kernel.org>
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On Wed, Mar 10, 2021 at 10:47 PM Nicolas Pitre <nico@fluxnic.net> wrote:
...
> > With CONFIG_TRIM_UNUSED_KSYMS=y I see a 3x-loops of building .version
> > and folowing steps - got no answer if this is intended.
>
> Yes it is intended. I explained it here:
>
> https://lkml.org/lkml/2021/3/9/1099
>

Ah, cool.
Thanks for that link.

> With CONFIG_TRIM_UNUSED_KSYMS some EXPORT_SYMBOL() are removed, which
> allows for optimizing away the corresponding code, which in turn opens
> the possibility for more EXPORT_SYMBOL() to be removed, etc. The process
> eventually converge to a stable build. Normally only 2 passes are needed
> to converge, but LTO opens the possibilities for extra passes.
>
> > Means longer build-time.
>
> Oh, absolutely.  LTO (at least when I played with it) is slow. Add the
> multi-pass from CONFIG_TRIM_UNUSED_KSYMS on top of that and your kernel
> build becomes agonizingly slow. This is not something you want when
> doing kernel development.
>

Thanks for the feedback.

> > I did not follow this anymore as both Kconfigs with Clang-LTO consume
> > more build-time and the resulting vmlinux is some MiB bigger than with
> > Clang-CFI.
>
> That's rather strange. At least with gcc LTO I always obtained smaller
> kernels.
>

I cannot say much to GCC-LTO - I never used it.

If you are interested in Clang-CFI (see [1]) - which requires
Clang-LTO enabled and LLVM/Clang >= 12.
Some hours ago version 12.0.0-rc3 was released, see [2].

- Sedat -

[1] https://github.com/samitolvanen/linux/commits/clang-cfi
[2] https://github.com/ClangBuiltLinux/linux/issues/1259