Date: Fri, 20 Nov 2020 08:23:11 -0800 From: Sami Tolvanen <samitolvanen@...gle.com> To: Nick Desaulniers <ndesaulniers@...gle.com> Cc: Masahiro Yamada <masahiroy@...nel.org>, Steven Rostedt <rostedt@...dmis.org>, Will Deacon <will@...nel.org>, Josh Poimboeuf <jpoimboe@...hat.com>, Peter Zijlstra <peterz@...radead.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, "Paul E. McKenney" <paulmck@...nel.org>, Kees Cook <keescook@...omium.org>, clang-built-linux <clang-built-linux@...glegroups.com>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, linux-arch <linux-arch@...r.kernel.org>, Linux ARM <linux-arm-kernel@...ts.infradead.org>, Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, linux-pci@...r.kernel.org Subject: Re: [PATCH v7 02/17] kbuild: add support for Clang LTO On Wed, Nov 18, 2020 at 3:49 PM Nick Desaulniers <ndesaulniers@...gle.com> wrote: > > On Wed, Nov 18, 2020 at 2:07 PM Sami Tolvanen <samitolvanen@...gle.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@...gle.com> > > Reviewed-by: Kees Cook <keescook@...omium.org> > > --- > > Makefile | 19 +++++++- > > arch/Kconfig | 75 +++++++++++++++++++++++++++++++ > > 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, 158 insertions(+), 18 deletions(-) > > > > diff --git a/Makefile b/Makefile > > index 8c8feb4245a6..240560e88d69 100644 > > --- a/Makefile > > +++ b/Makefile > > @@ -893,6 +893,21 @@ KBUILD_CFLAGS += $(CC_FLAGS_SCS) > > export CC_FLAGS_SCS > > endif > > > > +ifdef CONFIG_LTO_CLANG > > +ifdef CONFIG_THINLTO > > +CC_FLAGS_LTO += -flto=thin -fsplit-lto-unit > > +KBUILD_LDFLAGS += --thinlto-cache-dir=$(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 > > @@ -1473,7 +1488,7 @@ 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 > > # > > @@ -1719,7 +1734,7 @@ 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..a41fcb3ca7c6 100644 > > --- a/arch/Kconfig > > +++ b/arch/Kconfig > > @@ -598,6 +598,81 @@ 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 > > + > > + If you say N here, the compiler will use full LTO, which may > > + produce faster code, but building the kernel will be significantly > > + slower as the linker won't efficiently utilize multiple threads. > > + > > + If unsure, say Y. > > I think the order of these new configs makes it so that ThinLTO > appears above LTO in menuconfig; I don't like that, and wish it came > immediately after. Does `THINLTO` have to be defined _after_ the > choice for LTO_NONE/LTO_CLANG, perhaps? > > Secondly, I don't like how ThinLTO is a config and not a choice. If I > don't set ThinLTO, what am I getting? That's a rhetorical question; I > know its full LTO, and I guess the help text does talk about the > tradeoffs and what you would get. I guess what's curious to me is > "why does it display ThinLTO? Why not FullLTO?" I can't help but > wonder if a kconfig `choice` rather than a `config` would be better > here, that way it's more obvious the user is making a choice between > ThinLTO vs Full LTO, rather than the current patches which look like > "ThinkLTO on/off." Changing the ThinLTO config to a choice and moving it after the main LTO config sounds like a good idea to me. I'll see if I can change this in v8. Thanks! Sami
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