From: Masahiro Yamada <masahiroy@kernel.org>
To: Sami Tolvanen <samitolvanen@google.com>
Cc: Will Deacon <will@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Steven Rostedt <rostedt@goodmis.org>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
"Paul E. McKenney" <paulmck@kernel.org>,
Kees Cook <keescook@chromium.org>,
Nick Desaulniers <ndesaulniers@google.com>,
clang-built-linux <clang-built-linux@googlegroups.com>,
Kernel Hardening <kernel-hardening@lists.openwall.com>,
linux-arch <linux-arch@vger.kernel.org>,
linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
linux-pci@vger.kernel.org, X86 ML <x86@kernel.org>
Subject: Re: [PATCH v2 00/28] Add support for Clang LTO
Date: Thu, 10 Sep 2020 10:18:05 +0900 [thread overview]
Message-ID: <CAK7LNAR9zzP0ZU3b__PZv8gRtKrwz6-8GE1zG5UyFx1wDpOBzQ@mail.gmail.com> (raw)
In-Reply-To: <20200908234643.GF1060586@google.com>
On Wed, Sep 9, 2020 at 8:46 AM Sami Tolvanen <samitolvanen@google.com> wrote:
>
> On Sun, Sep 06, 2020 at 09:24:38AM +0900, Masahiro Yamada wrote:
> > On Fri, Sep 4, 2020 at 5:30 AM Sami Tolvanen <samitolvanen@google.com> wrote:
> > >
> > > This patch series adds support for building x86_64 and arm64 kernels
> > > with Clang's Link Time Optimization (LTO).
> > >
> > > In addition to performance, the primary motivation for LTO is
> > > to allow Clang's Control-Flow Integrity (CFI) to be used in the
> > > kernel. Google has shipped millions of Pixel devices running three
> > > major kernel versions with LTO+CFI since 2018.
> > >
> > > Most of the patches are build system changes for handling LLVM
> > > bitcode, which Clang produces with LTO instead of ELF object files,
> > > postponing ELF processing until a later stage, and ensuring initcall
> > > ordering.
> > >
> > > Note that patches 1-4 are not directly related to LTO, but are
> > > needed to compile LTO kernels with ToT Clang, so I'm including them
> > > in the series for your convenience:
> > >
> > > - Patches 1-3 are required for building the kernel with ToT Clang,
> > > and IAS, and patch 4 is needed to build allmodconfig with LTO.
> > >
> > > - Patches 3-4 are already in linux-next, but not yet in 5.9-rc.
> > >
> >
> >
> > I still do not understand how this patch set works.
> > (only me?)
> >
> > Please let me ask fundamental questions.
> >
> >
> >
> > I applied this series on top of Linus' tree,
> > and compiled for ARCH=arm64.
> >
> > I compared the kernel size with/without LTO.
> >
> >
> >
> > [1] No LTO (arm64 defconfig, CONFIG_LTO_NONE)
> >
> > $ llvm-size vmlinux
> > text data bss dec hex filename
> > 15848692 10099449 493060 26441201 19375f1 vmlinux
> >
> >
> >
> > [2] Clang LTO (arm64 defconfig + CONFIG_LTO_CLANG)
> >
> > $ llvm-size vmlinux
> > text data bss dec hex filename
> > 15906864 10197445 490804 26595113 195cf29 vmlinux
> >
> >
> > I compared the size of raw binary, arch/arm64/boot/Image.
> > Its size increased too.
> >
> >
> >
> > So, in my experiment, enabling CONFIG_LTO_CLANG
> > increases the kernel size.
> > Is this correct?
>
> Yes. LTO does produce larger binaries, mostly due to function
> inlining between translation units, I believe. The compiler people
> can probably give you a more detailed answer here. Without -mllvm
> -import-instr-limit, the binaries would be even larger.
>
> > One more thing, could you teach me
> > how Clang LTO optimizes the code against
> > relocatable objects?
> >
> >
> >
> > When I learned Clang LTO first, I read this document:
> > https://llvm.org/docs/LinkTimeOptimization.html
> >
> > It is easy to confirm the final executable
> > does not contain foo2, foo3...
> >
> >
> >
> > In contrast to userspace programs,
> > kernel modules are basically relocatable objects.
> >
> > Does Clang drop unused symbols from relocatable objects?
> > If so, how?
>
> I don't think the compiler can legally drop global symbols from
> relocatable objects, but it can rename and possibly even drop static
> functions.
Compilers can drop static functions without LTO.
Rather, it is a compiler warning
(-Wunused-function), so the code should be cleaned up.
> This is why we need global wrappers for initcalls, for
> example, to have stable symbol names.
>
> Sami
At first, I thought the motivation of LTO
was to remove unused global symbols, and
to perform further optimization.
It is true for userspace programs.
In fact, the example of
https://llvm.org/docs/LinkTimeOptimization.html
produces a smaller binary.
In contrast, this patch set produces a bigger kernel
because LTO cannot remove any unused symbol.
So, I do not understand what the benefit is.
Is inlining beneficial?
I am not sure.
Documentation/process/coding-style.rst
"15) The inline disease"
mentions that inlining is not always
a good thing.
As a whole, I still do not understand
the motivation of this patch set.
--
Best Regards
Masahiro Yamada
next prev parent reply other threads:[~2020-09-10 1:58 UTC|newest]
Thread overview: 207+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-24 20:31 [PATCH 00/22] add support for Clang LTO Sami Tolvanen
2020-06-24 20:31 ` [PATCH 01/22] objtool: use sh_info to find the base for .rela sections Sami Tolvanen
2020-06-24 20:31 ` [PATCH 02/22] kbuild: add support for Clang LTO Sami Tolvanen
2020-06-24 20:53 ` Nick Desaulniers
2020-06-24 21:29 ` Sami Tolvanen
2020-06-25 2:26 ` Nathan Chancellor
2020-06-25 16:13 ` Sami Tolvanen
2020-06-24 20:31 ` [PATCH 03/22] kbuild: lto: fix module versioning Sami Tolvanen
2020-06-24 20:31 ` [PATCH 04/22] kbuild: lto: fix recordmcount Sami Tolvanen
2020-06-24 21:27 ` Peter Zijlstra
2020-06-24 21:45 ` Sami Tolvanen
2020-06-25 7:45 ` Peter Zijlstra
2020-06-25 16:15 ` Sami Tolvanen
2020-06-25 20:02 ` [RFC][PATCH] objtool,x86_64: Replace recordmcount with objtool Peter Zijlstra
2020-06-25 20:54 ` Nick Desaulniers
2020-06-25 22:40 ` Sami Tolvanen
2020-06-26 11:29 ` Peter Zijlstra
2020-06-26 11:42 ` Peter Zijlstra
2020-07-17 17:28 ` Sami Tolvanen
2020-07-17 17:36 ` Steven Rostedt
2020-07-17 17:47 ` Sami Tolvanen
2020-07-17 18:05 ` Steven Rostedt
2020-07-20 16:52 ` Sami Tolvanen
2020-07-22 17:58 ` Steven Rostedt
2020-07-22 18:07 ` Sami Tolvanen
2020-07-22 17:55 ` Steven Rostedt
2020-07-22 18:41 ` Peter Zijlstra
2020-07-22 19:09 ` Steven Rostedt
2020-07-22 20:03 ` Sami Tolvanen
2020-07-22 23:56 ` Peter Zijlstra
2020-07-23 0:06 ` Steven Rostedt
2020-08-06 22:09 ` Sami Tolvanen
2020-06-24 20:31 ` [PATCH 05/22] kbuild: lto: postpone objtool Sami Tolvanen
2020-06-24 21:19 ` Peter Zijlstra
2020-06-24 21:49 ` Sami Tolvanen
2020-06-25 7:47 ` Peter Zijlstra
2020-06-25 16:22 ` Sami Tolvanen
2020-06-25 18:33 ` Peter Zijlstra
2020-06-25 19:32 ` Sami Tolvanen
2020-06-24 20:31 ` [PATCH 06/22] kbuild: lto: limit inlining Sami Tolvanen
2020-06-24 21:20 ` Peter Zijlstra
2020-06-24 23:37 ` Sami Tolvanen
2020-06-24 20:31 ` [PATCH 07/22] kbuild: lto: merge module sections Sami Tolvanen
2020-06-24 21:01 ` Nick Desaulniers
2020-06-24 21:31 ` Sami Tolvanen
2020-06-24 20:31 ` [PATCH 08/22] kbuild: lto: remove duplicate dependencies from .mod files Sami Tolvanen
2020-06-24 21:13 ` Nick Desaulniers
2020-06-24 20:31 ` [PATCH 09/22] init: lto: ensure initcall ordering Sami Tolvanen
2020-06-24 20:31 ` [PATCH 10/22] init: lto: fix PREL32 relocations Sami Tolvanen
2020-06-24 20:31 ` [PATCH 11/22] pci: " Sami Tolvanen
2020-07-17 20:26 ` Bjorn Helgaas
2020-07-22 18:15 ` Sami Tolvanen
2020-06-24 20:31 ` [PATCH 12/22] modpost: lto: strip .lto from module names Sami Tolvanen
2020-06-24 22:05 ` Nick Desaulniers
2020-06-24 20:31 ` [PATCH 13/22] scripts/mod: disable LTO for empty.c Sami Tolvanen
2020-06-24 20:57 ` Nick Desaulniers
2020-06-24 20:31 ` [PATCH 14/22] efi/libstub: disable LTO Sami Tolvanen
2020-06-24 20:31 ` [PATCH 15/22] drivers/misc/lkdtm: disable LTO for rodata.o Sami Tolvanen
2020-06-24 20:31 ` [PATCH 16/22] arm64: export CC_USING_PATCHABLE_FUNCTION_ENTRY Sami Tolvanen
2020-06-24 20:31 ` [PATCH 17/22] arm64: vdso: disable LTO Sami Tolvanen
2020-06-24 20:58 ` Nick Desaulniers
2020-06-24 21:09 ` Nick Desaulniers
2020-06-24 23:51 ` Andi Kleen
2020-06-24 21:52 ` Sami Tolvanen
2020-06-24 23:05 ` Nick Desaulniers
2020-06-24 23:39 ` Sami Tolvanen
2020-06-24 20:31 ` [PATCH 18/22] arm64: allow LTO_CLANG and THINLTO to be selected Sami Tolvanen
2020-06-24 20:31 ` [PATCH 19/22] x86, vdso: disable LTO only for vDSO Sami Tolvanen
2020-06-24 20:31 ` [PATCH 20/22] x86, ftrace: disable recordmcount for ftrace_make_nop Sami Tolvanen
2020-06-24 20:31 ` [PATCH 21/22] x86, relocs: Ignore L4_PAGE_OFFSET relocations Sami Tolvanen
2020-06-24 20:32 ` [PATCH 22/22] x86, build: allow LTO_CLANG and THINLTO to be selected Sami Tolvanen
2020-06-24 21:15 ` [PATCH 00/22] add support for Clang LTO Peter Zijlstra
2020-06-24 21:30 ` Sami Tolvanen
2020-06-25 8:27 ` Will Deacon
2020-06-24 21:31 ` Nick Desaulniers
2020-06-25 8:03 ` Peter Zijlstra
2020-06-25 8:24 ` Peter Zijlstra
2020-06-25 8:57 ` Peter Zijlstra
2020-06-30 19:19 ` Marco Elver
2020-06-30 20:12 ` Peter Zijlstra
2020-06-30 20:30 ` Paul E. McKenney
2020-07-01 9:10 ` Peter Zijlstra
2020-07-01 14:20 ` David Laight
2020-07-01 16:06 ` Paul E. McKenney
2020-07-02 9:37 ` David Laight
2020-07-02 18:00 ` Paul E. McKenney
2020-07-01 9:41 ` Marco Elver
2020-07-01 10:03 ` Will Deacon
2020-07-01 11:40 ` Peter Zijlstra
2020-07-01 14:06 ` Paul E. McKenney
2020-07-01 15:05 ` Peter Zijlstra
2020-07-01 16:03 ` Paul E. McKenney
2020-07-02 8:20 ` Peter Zijlstra
2020-07-02 17:59 ` Paul E. McKenney
2020-07-03 13:13 ` Peter Zijlstra
2020-07-03 13:25 ` Peter Zijlstra
2020-07-03 14:51 ` Paul E. McKenney
2020-07-03 14:42 ` Paul E. McKenney
2020-07-06 16:26 ` Paul E. McKenney
2020-07-06 18:29 ` Peter Zijlstra
2020-07-06 18:39 ` Paul E. McKenney
2020-07-06 19:40 ` Peter Zijlstra
2020-07-06 23:41 ` Paul E. McKenney
2020-06-28 16:56 ` Masahiro Yamada
2020-06-29 23:20 ` Sami Tolvanen
2020-07-07 15:51 ` Sami Tolvanen
2020-07-07 16:05 ` Sami Tolvanen
2020-07-07 16:56 ` Jakub Kicinski
2020-07-07 17:17 ` Nick Desaulniers
2020-07-07 17:30 ` Jakub Kicinski
2020-07-11 16:32 ` Paul Menzel
2020-07-12 8:59 ` Sedat Dilek
2020-07-12 18:40 ` Nathan Chancellor
2020-07-14 9:44 ` Sedat Dilek
2020-07-14 17:54 ` Nick Desaulniers
2020-07-12 23:34 ` Sami Tolvanen
2020-07-14 12:16 ` Paul Menzel
2020-07-14 12:35 ` Sedat Dilek
2020-09-03 20:30 ` [PATCH v2 00/28] Add " Sami Tolvanen
2020-09-03 20:30 ` [PATCH v2 01/28] x86/boot/compressed: Disable relocation relaxation Sami Tolvanen
2020-09-03 21:44 ` Kees Cook
2020-09-03 23:42 ` Arvind Sankar
2020-09-04 7:14 ` Nathan Chancellor
2020-09-03 20:30 ` [PATCH v2 02/28] x86/asm: Replace __force_order with memory clobber Sami Tolvanen
2020-09-03 21:45 ` Kees Cook
2020-09-03 20:30 ` [PATCH v2 03/28] lib/string.c: implement stpcpy Sami Tolvanen
2020-09-03 21:47 ` Kees Cook
2020-09-03 20:30 ` [PATCH v2 04/28] RAS/CEC: Fix cec_init() prototype Sami Tolvanen
2020-09-03 21:50 ` Kees Cook
2020-09-03 20:30 ` [PATCH v2 05/28] objtool: Add a pass for generating __mcount_loc Sami Tolvanen
2020-09-03 21:51 ` Kees Cook
2020-09-03 22:03 ` Sami Tolvanen
2020-09-04 9:31 ` peterz
2020-09-10 18:29 ` Kees Cook
2020-09-03 20:30 ` [PATCH v2 06/28] objtool: Don't autodetect vmlinux.o Sami Tolvanen
2020-09-03 21:52 ` Kees Cook
2020-09-03 20:30 ` [PATCH v2 07/28] kbuild: add support for objtool mcount Sami Tolvanen
2020-09-03 21:56 ` Kees Cook
2020-09-03 20:30 ` [PATCH v2 08/28] x86, build: use " Sami Tolvanen
2020-09-03 21:58 ` Kees Cook
2020-09-03 22:11 ` Sami Tolvanen
2020-09-03 20:30 ` [PATCH v2 09/28] kbuild: add support for Clang LTO Sami Tolvanen
2020-09-03 22:08 ` Kees Cook
2020-09-08 17:02 ` Sami Tolvanen
2020-09-05 19:36 ` Masahiro Yamada
2020-09-08 17:10 ` Sami Tolvanen
2020-09-05 20:17 ` Masahiro Yamada
2020-09-08 17:14 ` Sami Tolvanen
2020-09-07 15:30 ` Masahiro Yamada
2020-09-08 17:30 ` Sami Tolvanen
2020-09-03 20:30 ` [PATCH v2 10/28] kbuild: lto: fix module versioning Sami Tolvanen
2020-09-03 22:11 ` Kees Cook
2020-09-08 18:23 ` Sami Tolvanen
2020-09-03 20:30 ` [PATCH v2 11/28] kbuild: lto: postpone objtool Sami Tolvanen
2020-09-03 22:19 ` Kees Cook
2020-09-08 20:56 ` Sami Tolvanen
2020-09-03 20:30 ` [PATCH v2 12/28] kbuild: lto: limit inlining Sami Tolvanen
2020-09-03 22:20 ` Kees Cook
2020-09-03 20:30 ` [PATCH v2 13/28] kbuild: lto: merge module sections Sami Tolvanen
2020-09-03 22:23 ` Kees Cook
2020-09-07 15:25 ` Masahiro Yamada
2020-09-08 21:07 ` Sami Tolvanen
2020-09-03 20:30 ` [PATCH v2 14/28] kbuild: lto: remove duplicate dependencies from .mod files Sami Tolvanen
2020-09-03 22:29 ` Kees Cook
2020-09-03 20:30 ` [PATCH v2 15/28] init: lto: ensure initcall ordering Sami Tolvanen
2020-09-03 22:40 ` Kees Cook
2020-09-08 21:16 ` Sami Tolvanen
2020-09-10 9:25 ` David Woodhouse
2020-09-10 15:07 ` Sami Tolvanen
2020-09-03 20:30 ` [PATCH v2 16/28] init: lto: fix PREL32 relocations Sami Tolvanen
2020-09-03 22:41 ` Kees Cook
2020-09-03 20:30 ` [PATCH v2 17/28] PCI: Fix PREL32 relocations for LTO Sami Tolvanen
2020-09-03 22:42 ` Kees Cook
2020-09-03 20:30 ` [PATCH v2 18/28] modpost: lto: strip .lto from module names Sami Tolvanen
2020-09-03 22:42 ` Kees Cook
2020-09-03 20:30 ` [PATCH v2 19/28] scripts/mod: disable LTO for empty.c Sami Tolvanen
2020-09-03 22:43 ` Kees Cook
2020-09-03 20:30 ` [PATCH v2 20/28] efi/libstub: disable LTO Sami Tolvanen
2020-09-03 22:43 ` Kees Cook
2020-09-03 20:30 ` [PATCH v2 21/28] drivers/misc/lkdtm: disable LTO for rodata.o Sami Tolvanen
2020-09-03 20:30 ` [PATCH v2 22/28] arm64: export CC_USING_PATCHABLE_FUNCTION_ENTRY Sami Tolvanen
2020-09-03 22:44 ` Kees Cook
2020-09-08 21:23 ` Sami Tolvanen
2020-09-03 20:30 ` [PATCH v2 23/28] arm64: vdso: disable LTO Sami Tolvanen
2020-09-03 22:45 ` Kees Cook
2020-09-03 20:30 ` [PATCH v2 24/28] KVM: arm64: disable LTO for the nVHE directory Sami Tolvanen
2020-09-03 22:45 ` Kees Cook
2020-09-03 20:30 ` [PATCH v2 25/28] arm64: allow LTO_CLANG and THINLTO to be selected Sami Tolvanen
2020-09-03 22:45 ` Kees Cook
2020-09-03 20:30 ` [PATCH v2 26/28] x86, vdso: disable LTO only for vDSO Sami Tolvanen
2020-09-03 22:46 ` Kees Cook
2020-09-03 20:30 ` [PATCH v2 27/28] x86, relocs: Ignore L4_PAGE_OFFSET relocations Sami Tolvanen
2020-09-03 22:47 ` Kees Cook
2020-09-08 23:28 ` Sami Tolvanen
2020-09-03 20:30 ` [PATCH v2 28/28] x86, build: allow LTO_CLANG and THINLTO to be selected Sami Tolvanen
2020-09-03 22:48 ` Kees Cook
2020-09-03 23:34 ` [PATCH v2 00/28] Add support for Clang LTO Kees Cook
2020-09-04 4:45 ` Nathan Chancellor
2020-09-03 23:38 ` Kees Cook
2020-09-04 7:53 ` Sedat Dilek
2020-09-04 8:55 ` peterz
2020-09-04 9:08 ` Sedat Dilek
2020-09-06 0:24 ` Masahiro Yamada
2020-09-08 23:46 ` Sami Tolvanen
2020-09-10 1:18 ` Masahiro Yamada [this message]
2020-09-10 15:17 ` Sami Tolvanen
2020-09-10 18:18 ` Kees Cook
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