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From: Kees Cook <keescook@chromium.org>
To: Sami Tolvanen <samitolvanen@google.com>
Cc: Masahiro Yamada <masahiroy@kernel.org>,
	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>,
	Nick Desaulniers <ndesaulniers@google.com>,
	clang-built-linux@googlegroups.com,
	kernel-hardening@lists.openwall.com, linux-arch@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-pci@vger.kernel.org, x86@kernel.org
Subject: Re: [PATCH v2 09/28] kbuild: add support for Clang LTO
Date: Thu, 3 Sep 2020 15:08:59 -0700
Message-ID: <202009031504.07098D6F8@keescook> (raw)
In-Reply-To: <20200903203053.3411268-10-samitolvanen@google.com>

On Thu, Sep 03, 2020 at 01:30:34PM -0700, Sami Tolvanen wrote:
> This change adds build system support for Clang's Link Time
> Optimization (LTO). With -flto, instead of ELF object files, Clang
> produces LLVM bitcode, which is compiled into native code at link
> time, allowing the final binary to be optimized globally. For more
> details, see:
> 
>   https://llvm.org/docs/LinkTimeOptimization.html
> 
> The Kconfig option CONFIG_LTO_CLANG is implemented as a choice,
> which defaults to LTO being disabled. To use LTO, the architecture
> must select ARCH_SUPPORTS_LTO_CLANG and support:
> 
>   - compiling with Clang,
>   - compiling inline assembly with Clang's integrated assembler,
>   - and linking with LLD.
> 
> While using full LTO results in the best runtime performance, the
> compilation is not scalable in time or memory. CONFIG_THINLTO
> enables ThinLTO, which allows parallel optimization and faster
> incremental builds. ThinLTO is used by default if the architecture
> also selects ARCH_SUPPORTS_THINLTO:
> 
>   https://clang.llvm.org/docs/ThinLTO.html
> 
> To enable LTO, LLVM tools must be used to handle bitcode files. The
> easiest way is to pass the LLVM=1 option to make:
> 
>   $ make LLVM=1 defconfig
>   $ scripts/config -e LTO_CLANG
>   $ make LLVM=1
> 
> Alternatively, at least the following LLVM tools must be used:
> 
>   CC=clang LD=ld.lld AR=llvm-ar NM=llvm-nm
> 
> To prepare for LTO support with other compilers, common parts are
> gated behind the CONFIG_LTO option, and LTO can be disabled for
> specific files by filtering out CC_FLAGS_LTO.
> 
> Note that support for DYNAMIC_FTRACE and MODVERSIONS are added in
> follow-up patches.
> 
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>

I remain crazy excited about being able to use this in upstream. :)

The only suggestion I have here, if it might help with clarity, would be
to remove DISABLE_LTO globally as a separate patch, since it's entirely
unused in the kernel right now. This series removes it as it goes, which
I think is fine, but it might cause some reviewers to ponder "what's
this DISABLE_LTO thing? Don't we need that?" without realizing currently
unused in the kernel.

I'm glad to see the general CONFIG_LTO, as I think it should be easy for
GCC LTO support to get added when someone steps up to do it. The bulk of
the changed needed to support GCC LTO are part of this series already,
since the build problems involving non-ELF .o files and init ordering
are shared by Clang and GCC AFAICT.

Reviewed-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook

  reply index

Thread overview: 213+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-06-24 20:31 [PATCH 00/22] " 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
2020-06-25  0:58   ` kernel test robot
2020-06-25  4:19   ` kernel test robot
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
2020-06-24 22:49   ` kernel test robot
2020-06-24 23:03     ` Nick Desaulniers
2020-06-24 23:21       ` 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
2020-07-14 13:40         ` Paul Menzel
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 [this message]
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
2020-09-10 15:17         ` Sami Tolvanen
2020-09-10 18:18         ` Kees Cook

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