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From: Masahiro Yamada <masahiroy@kernel.org>
To: Sami Tolvanen <samitolvanen@google.com>
Cc: Steven Rostedt <rostedt@goodmis.org>,
	Will Deacon <will@kernel.org>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Peter Zijlstra <peterz@infradead.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
Subject: Re: [PATCH v8 02/16] kbuild: add support for Clang LTO
Date: Wed, 2 Dec 2020 11:59:37 +0900	[thread overview]
Message-ID: <CAK7LNASMh1KysAB4+gU7_iuTW+5GT2_yMDevwpLwx0iqjxwmWw@mail.gmail.com> (raw)
In-Reply-To: <20201201213707.541432-3-samitolvanen@google.com>

On Wed, Dec 2, 2020 at 6:37 AM 'Sami Tolvanen' via Clang Built Linux
<clang-built-linux@googlegroups.com> 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.
>
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> ---
>  Makefile                          | 19 ++++++-
>  arch/Kconfig                      | 88 +++++++++++++++++++++++++++++++
>  include/asm-generic/vmlinux.lds.h | 11 ++--
>  scripts/Makefile.build            |  9 +++-
>  scripts/Makefile.modfinal         |  9 +++-
>  scripts/Makefile.modpost          | 21 +++++++-
>  scripts/link-vmlinux.sh           | 32 ++++++++---
>  7 files changed, 171 insertions(+), 18 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index 16b7f0890e75..f5cac2428efc 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -891,6 +891,21 @@ KBUILD_CFLAGS      += $(CC_FLAGS_SCS)
>  export CC_FLAGS_SCS
>  endif
>
> +ifdef CONFIG_LTO_CLANG
> +ifdef CONFIG_LTO_CLANG_THIN
> +CC_FLAGS_LTO   += -flto=thin -fsplit-lto-unit
> +KBUILD_LDFLAGS += --thinlto-cache-dir=$(extmod-prefix).thinlto-cache
> +else
> +CC_FLAGS_LTO   += -flto
> +endif
> +CC_FLAGS_LTO   += -fvisibility=default
> +endif
> +
> +ifdef CONFIG_LTO
> +KBUILD_CFLAGS  += $(CC_FLAGS_LTO)
> +export CC_FLAGS_LTO
> +endif
> +
>  ifdef CONFIG_DEBUG_FORCE_FUNCTION_ALIGN_32B
>  KBUILD_CFLAGS += -falign-functions=32
>  endif
> @@ -1471,7 +1486,7 @@ MRPROPER_FILES += include/config include/generated          \
>                   *.spec
>
>  # Directories & files removed with 'make distclean'
> -DISTCLEAN_FILES += tags TAGS cscope* GPATH GTAGS GRTAGS GSYMS
> +DISTCLEAN_FILES += tags TAGS cscope* GPATH GTAGS GRTAGS GSYMS .thinlto-cache
>
>  # clean - Delete most, but leave enough to build external modules
>  #
> @@ -1717,7 +1732,7 @@ PHONY += compile_commands.json
>
>  clean-dirs := $(KBUILD_EXTMOD)
>  clean: rm-files := $(KBUILD_EXTMOD)/Module.symvers $(KBUILD_EXTMOD)/modules.nsdeps \
> -       $(KBUILD_EXTMOD)/compile_commands.json
> +       $(KBUILD_EXTMOD)/compile_commands.json $(KBUILD_EXTMOD)/.thinlto-cache
>
>  PHONY += help
>  help:
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 56b6ccc0e32d..30907b554451 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -598,6 +598,94 @@ config SHADOW_CALL_STACK
>           reading and writing arbitrary memory may be able to locate them
>           and hijack control flow by modifying the stacks.
>
> +config LTO
> +       bool
> +       help
> +         Selected if the kernel will be built using the compiler's LTO feature.
> +
> +config LTO_CLANG
> +       bool
> +       select LTO
> +       help
> +         Selected if the kernel will be built using Clang's LTO feature.
> +
> +config ARCH_SUPPORTS_LTO_CLANG
> +       bool
> +       help
> +         An architecture should select this option if it supports:
> +         - compiling with Clang,
> +         - compiling inline assembly with Clang's integrated assembler,
> +         - and linking with LLD.
> +
> +config ARCH_SUPPORTS_LTO_CLANG_THIN
> +       bool
> +       help
> +         An architecture should select this option if it can support Clang's
> +         ThinLTO mode.
> +
> +config HAS_LTO_CLANG
> +       def_bool y
> +       # Clang >= 11: https://github.com/ClangBuiltLinux/linux/issues/510
> +       depends on CC_IS_CLANG && CLANG_VERSION >= 110000 && LD_IS_LLD
> +       depends on $(success,$(NM) --help | head -n 1 | grep -qi llvm)
> +       depends on $(success,$(AR) --help | head -n 1 | grep -qi llvm)
> +       depends on ARCH_SUPPORTS_LTO_CLANG
> +       depends on !FTRACE_MCOUNT_USE_RECORDMCOUNT
> +       depends on !KASAN
> +       depends on !GCOV_KERNEL
> +       depends on !MODVERSIONS
> +       help
> +         The compiler and Kconfig options support building with Clang's
> +         LTO.
> +
> +choice
> +       prompt "Link Time Optimization (LTO)"
> +       default LTO_NONE
> +       help
> +         This option enables Link Time Optimization (LTO), which allows the
> +         compiler to optimize binaries globally.
> +
> +         If unsure, select LTO_NONE. Note that LTO is very resource-intensive
> +         so it's disabled by default.
> +
> +config LTO_NONE
> +       bool "None"
> +       help
> +         Build the kernel normally, without Link Time Optimization (LTO).
> +
> +config LTO_CLANG_FULL
> +       bool "Clang Full LTO (EXPERIMENTAL)"
> +       depends on HAS_LTO_CLANG
> +       select LTO_CLANG
> +       help
> +          This option enables Clang's full Link Time Optimization (LTO), which
> +          allows the compiler to optimize the kernel globally. If you enable
> +          this option, the compiler generates LLVM bitcode instead of ELF
> +          object files, and the actual compilation from bitcode happens at
> +          the LTO link step, which may take several minutes depending on the
> +          kernel configuration. More information can be found from LLVM's
> +          documentation:
> +
> +           https://llvm.org/docs/LinkTimeOptimization.html
> +

This help document is misleading.
People who read the document would misunderstand how great this feature would.

This should be added in the commit log and Kconfig help:

            In contrast to the example in the documentation, Clang LTO
            for the kernel cannot remove any unreachable function or data.
            In fact, this results in even bigger vmlinux and modules.




-- 
Best Regards
Masahiro Yamada

  reply	other threads:[~2020-12-02  3:01 UTC|newest]

Thread overview: 48+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-01 21:36 [PATCH v8 00/16] Add support for Clang LTO Sami Tolvanen
2020-12-01 21:36 ` [PATCH v8 01/16] tracing: move function tracer options to Kconfig Sami Tolvanen
2020-12-01 21:47   ` Steven Rostedt
2020-12-01 21:36 ` [PATCH v8 02/16] kbuild: add support for Clang LTO Sami Tolvanen
2020-12-02  2:59   ` Masahiro Yamada [this message]
2020-12-03  0:07   ` Nick Desaulniers
2020-12-01 21:36 ` [PATCH v8 03/16] kbuild: lto: fix module versioning Sami Tolvanen
2020-12-01 21:36 ` [PATCH v8 04/16] kbuild: lto: limit inlining Sami Tolvanen
2020-12-01 21:36 ` [PATCH v8 05/16] kbuild: lto: merge module sections Sami Tolvanen
2020-12-01 21:36 ` [PATCH v8 06/16] kbuild: lto: remove duplicate dependencies from .mod files Sami Tolvanen
2020-12-01 21:36 ` [PATCH v8 07/16] init: lto: ensure initcall ordering Sami Tolvanen
2020-12-01 21:36 ` [PATCH v8 08/16] init: lto: fix PREL32 relocations Sami Tolvanen
2020-12-01 21:37 ` [PATCH v8 09/16] PCI: Fix PREL32 relocations for LTO Sami Tolvanen
2020-12-01 21:37 ` [PATCH v8 10/16] modpost: lto: strip .lto from module names Sami Tolvanen
2020-12-01 21:37 ` [PATCH v8 11/16] scripts/mod: disable LTO for empty.c Sami Tolvanen
2020-12-01 21:37 ` [PATCH v8 12/16] efi/libstub: disable LTO Sami Tolvanen
2020-12-01 21:37 ` [PATCH v8 13/16] drivers/misc/lkdtm: disable LTO for rodata.o Sami Tolvanen
2020-12-01 21:37 ` [PATCH v8 14/16] arm64: vdso: disable LTO Sami Tolvanen
2020-12-01 21:37 ` [PATCH v8 15/16] arm64: disable recordmcount with DYNAMIC_FTRACE_WITH_REGS Sami Tolvanen
2020-12-01 21:37 ` [PATCH v8 16/16] arm64: allow LTO to be selected Sami Tolvanen
2020-12-03  0:01 ` [PATCH v8 00/16] Add support for Clang LTO Nick Desaulniers
2020-12-03 11:26 ` Will Deacon
2020-12-03 17:07   ` Sami Tolvanen
2020-12-03 18:21     ` Nathan Chancellor
2020-12-03 18:22     ` Will Deacon
2020-12-03 22:32       ` Nick Desaulniers
2020-12-04  9:35         ` Will Deacon
2020-12-04 22:52         ` Sami Tolvanen
2020-12-06  6:50           ` Nathan Chancellor
2020-12-06 20:09             ` Sami Tolvanen
2020-12-08  0:46               ` Nathan Chancellor
2020-12-08 12:15 ` Arnd Bergmann
2020-12-08 13:54   ` Arnd Bergmann
2020-12-08 16:53     ` Sami Tolvanen
2020-12-08 16:56       ` Arnd Bergmann
2020-12-08 16:43   ` Sami Tolvanen
     [not found]     ` <CAK8P3a1Xfpt7QLkvxjtXKcgzcWkS8g9bmxD687+rqjTafTzKrg@mail.gmail.com>
2020-12-08 21:09       ` Nick Desaulniers
2020-12-08 22:20         ` Arnd Bergmann
2020-12-09 16:11           ` Sami Tolvanen
     [not found]       ` <CAK8P3a3O65m6Us=YvCP3QA+0kqAeEqfi-DLOJa+JYmBqs8-JcA@mail.gmail.com>
2020-12-09  5:23         ` Fāng-ruì Sòng
2020-12-09  9:07           ` Arnd Bergmann
2020-12-09 16:09         ` Sami Tolvanen
2020-12-09 19:24           ` Arnd Bergmann
2020-12-09  4:55     ` Fangrui Song
2020-12-09  9:19       ` Arnd Bergmann
2020-12-09 12:35   ` Arnd Bergmann
2020-12-09 16:25     ` Sami Tolvanen
2020-12-09 17:51       ` Arnd Bergmann

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