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From: Sami Tolvanen <samitolvanen@google.com>
To: Nathan Chancellor <natechancellor@gmail.com>
Cc: Masahiro Yamada <masahiroy@kernel.org>,
	Will Deacon <will@kernel.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@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 02/22] kbuild: add support for Clang LTO
Date: Thu, 25 Jun 2020 09:13:39 -0700
Message-ID: <20200625161339.GA173089@google.com> (raw)
In-Reply-To: <20200625022647.GB2871607@ubuntu-n2-xlarge-x86>

On Wed, Jun 24, 2020 at 07:26:47PM -0700, Nathan Chancellor wrote:
> Hi Sami,
> 
> On Wed, Jun 24, 2020 at 01:31:40PM -0700, 'Sami Tolvanen' via Clang Built Linux 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>
> > ---
> >  Makefile                          | 16 ++++++++
> >  arch/Kconfig                      | 66 +++++++++++++++++++++++++++++++
> >  include/asm-generic/vmlinux.lds.h | 11 ++++--
> >  scripts/Makefile.build            |  9 ++++-
> >  scripts/Makefile.modfinal         |  9 ++++-
> >  scripts/Makefile.modpost          | 24 ++++++++++-
> >  scripts/link-vmlinux.sh           | 32 +++++++++++----
> >  7 files changed, 151 insertions(+), 16 deletions(-)
> > 
> > diff --git a/Makefile b/Makefile
> > index ac2c61c37a73..0c7fe6fb2143 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -886,6 +886,22 @@ KBUILD_CFLAGS	+= $(CC_FLAGS_SCS)
> >  export CC_FLAGS_SCS
> >  endif
> >  
> > +ifdef CONFIG_LTO_CLANG
> > +ifdef CONFIG_THINLTO
> > +CC_FLAGS_LTO_CLANG := -flto=thin $(call cc-option, -fsplit-lto-unit)
> > +KBUILD_LDFLAGS	+= --thinlto-cache-dir=.thinlto-cache
> > +else
> > +CC_FLAGS_LTO_CLANG := -flto
> > +endif
> > +CC_FLAGS_LTO_CLANG += -fvisibility=default
> > +endif
> > +
> > +ifdef CONFIG_LTO
> > +CC_FLAGS_LTO	:= $(CC_FLAGS_LTO_CLANG)
> > +KBUILD_CFLAGS	+= $(CC_FLAGS_LTO)
> > +export CC_FLAGS_LTO
> > +endif
> > +
> >  # arch Makefile may override CC so keep this after arch Makefile is included
> >  NOSTDINC_FLAGS += -nostdinc -isystem $(shell $(CC) -print-file-name=include)
> >  
> > diff --git a/arch/Kconfig b/arch/Kconfig
> > index 8cc35dc556c7..e00b122293f8 100644
> > --- a/arch/Kconfig
> > +++ b/arch/Kconfig
> > @@ -552,6 +552,72 @@ 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
> > +
> > +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_THINLTO
> > +	bool
> > +	help
> > +	  An architecture should select this option if it supports Clang's
> > +	  ThinLTO.
> > +
> > +config THINLTO
> > +	bool "Clang ThinLTO"
> > +	depends on LTO_CLANG && ARCH_SUPPORTS_THINLTO
> > +	default y
> > +	help
> > +	  This option enables Clang's ThinLTO, which allows for parallel
> > +	  optimization and faster incremental compiles. More information
> > +	  can be found from Clang's documentation:
> > +
> > +	    https://clang.llvm.org/docs/ThinLTO.html
> > +
> > +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.
> > +
> > +config LTO_NONE
> > +	bool "None"
> > +
> > +config LTO_CLANG
> > +	bool "Clang's Link Time Optimization (EXPERIMENTAL)"
> > +	depends on CC_IS_CLANG && CLANG_VERSION >= 110000 && LD_IS_LLD
> 
> I am curious, what is the reason for gating this at clang 11.0.0?
> 
> Presumably this? https://github.com/ClangBuiltLinux/linux/issues/510
> 
> It might be nice to notate this so that we do not have to wonder :)

Yes, that's the reason. I'll add a note about it. Thanks!

Sami

  reply index

Thread overview: 118+ 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 [this message]
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

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