Re: [PATCH] [RFC] arm64: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

From: Nicolas Pitre <nico@fluxnic.net>
To: Sedat Dilek <sedat.dilek@gmail.com>
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>
Subject: Re: [PATCH] [RFC] arm64: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION
Date: Wed, 10 Mar 2021 16:47:49 -0500 (EST)	[thread overview]
Message-ID: <ros2o1p7-r65q-n4p-3o38-oo36rr58q041@syhkavp.arg> (raw)
In-Reply-To: <CA+icZUWk-9i8BSf70qE_9f=mekscQ063q+aUKzSNBymNLAbcTA@mail.gmail.com>

On Wed, 10 Mar 2021, Sedat Dilek wrote:

> The best results on size-reduction of vmlinux I got with Clang-CFI on x86-64.
> 
> Clang-LTO and Clang-CFI:
> I was able to build with CONFIG_TRIM_UNUSED_KSYMS=y which needs to add
> a whitelist file or add a whitelist to scripts/gen_autoksyms.sh.
> And boot on bare metal.
> Furthermore, I was able to compile
> CONFIG_HAVE_LD_DEAD_CODE_DATA_ELIMINATION=y with and without
> CONFIG_TRIM_UNUSED_KSYMS=y.
> Every kernel I had CONFIG_HAVE_LD_DEAD_CODE_DATA_ELIMINATION=y does not boot.
> Yes, there is a size reduction with both enabled but not that good as
> with Clang-CFI.
> All testings with several iterations of LLVM/Clang v13-git.
> 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

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.

> 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.

Nicolas