On Wed, Nov 18, 2020 at 2:07 PM Sami Tolvanen <samitolvanen@xxxxxxxxxx> wrote: > > This patch series adds support for building the kernel 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 v7 brings back arm64 support as Will has now staged the > prerequisite memory ordering patches [1], and drops x86_64 while we work > on fixing the remaining objtool warnings [2]. > > [1] https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/log/?h=for-next/lto > [2] https://lore.kernel.org/lkml/20201114004911.aip52eimk6c2uxd4@treble/ > > You can also pull this series from > > https://github.com/samitolvanen/linux.git lto-v7 Thanks for continuing to drive this series Sami. For the series, Tested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> I did virtualized boot tests with the series applied to aarch64 defconfig without CONFIG_LTO, with CONFIG_LTO_CLANG, and a third time with CONFIG_THINLTO. If you make changes to the series in follow ups, please drop my tested by tag from the modified patches and I'll help re-test. Some minor feedback on the Kconfig change, but I'll post it off of that patch. > > --- > Changes in v7: > > - Rebased to master again. > > - Added back arm64 patches as the prerequisites are now staged, > and dropped x86_64 support until the remaining objtool issues > are resolved. > > - Dropped ifdefs from module.lds.S. > > Changes in v6: > > - Added the missing --mcount flag to patch 5. > > - Dropped the arm64 patches from this series and will repost them > later. > > Changes in v5: > > - Rebased on top of tip/master. > > - Changed the command line for objtool to use --vmlinux --duplicate > to disable warnings about retpoline thunks and to fix .orc_unwind > generation for vmlinux.o. > > - Added --noinstr flag to objtool, so we can use --vmlinux without > also enabling noinstr validation. > > - Disabled objtool's unreachable instruction warnings with LTO to > disable false positives for the int3 padding in vmlinux.o. > > - Added ANNOTATE_RETPOLINE_SAFE annotations to the indirect jumps > in x86 assembly code to fix objtool warnings with retpoline. > > - Fixed modpost warnings about missing version information with > CONFIG_MODVERSIONS. > > - Included Makefile.lib into Makefile.modpost for ld_flags. Thanks > to Sedat for pointing this out. > > - Updated the help text for ThinLTO to better explain the trade-offs. > > - Updated commit messages with better explanations. > > Changes in v4: > > - Fixed a typo in Makefile.lib to correctly pass --no-fp to objtool. > > - Moved ftrace configs related to generating __mcount_loc to Kconfig, > so they are available also in Makefile.modfinal. > > - Dropped two prerequisite patches that were merged to Linus' tree. > > Changes in v3: > > - Added a separate patch to remove the unused DISABLE_LTO treewide, > as filtering out CC_FLAGS_LTO instead is preferred. > > - Updated the Kconfig help to explain why LTO is behind a choice > and disabled by default. > > - Dropped CC_FLAGS_LTO_CLANG, compiler-specific LTO flags are now > appended directly to CC_FLAGS_LTO. > > - Updated $(AR) flags as KBUILD_ARFLAGS was removed earlier. > > - Fixed ThinLTO cache handling for external module builds. > > - Rebased on top of Masahiro's patch for preprocessing modules.lds, > and moved the contents of module-lto.lds to modules.lds.S. > > - Moved objtool_args to Makefile.lib to avoid duplication of the > command line parameters in Makefile.modfinal. > > - Clarified in the commit message for the initcall ordering patch > that the initcall order remains the same as without LTO. > > - Changed link-vmlinux.sh to use jobserver-exec to control the > number of jobs started by generate_initcall_ordering.pl. > > - Dropped the x86/relocs patch to whitelist L4_PAGE_OFFSET as it's > no longer needed with ToT kernel. > > - Disabled LTO for arch/x86/power/cpu.c to work around a Clang bug > with stack protector attributes. > > Changes in v2: > > - Fixed -Wmissing-prototypes warnings with W=1. > > - Dropped cc-option from -fsplit-lto-unit and added .thinlto-cache > scrubbing to make distclean. > > - Added a comment about Clang >=11 being required. > > - Added a patch to disable LTO for the arm64 KVM nVHE code. > > - Disabled objtool's noinstr validation with LTO unless enabled. > > - Included Peter's proposed objtool mcount patch in the series > and replaced recordmcount with the objtool pass to avoid > whitelisting relocations that are not calls. > > - Updated several commit messages with better explanations. > > > Sami Tolvanen (17): > tracing: move function tracer options to Kconfig > kbuild: add support for Clang LTO > kbuild: lto: fix module versioning > kbuild: lto: limit inlining > kbuild: lto: merge module sections > kbuild: lto: remove duplicate dependencies from .mod files > init: lto: ensure initcall ordering > init: lto: fix PREL32 relocations > PCI: Fix PREL32 relocations for LTO > modpost: lto: strip .lto from module names > scripts/mod: disable LTO for empty.c > efi/libstub: disable LTO > drivers/misc/lkdtm: disable LTO for rodata.o > arm64: vdso: disable LTO > KVM: arm64: disable LTO for the nVHE directory > arm64: disable recordmcount with DYNAMIC_FTRACE_WITH_REGS > arm64: allow LTO_CLANG and THINLTO to be selected > > .gitignore | 1 + > Makefile | 45 +++-- > arch/Kconfig | 74 +++++++ > arch/arm64/Kconfig | 4 + > arch/arm64/kernel/vdso/Makefile | 3 +- > arch/arm64/kvm/hyp/nvhe/Makefile | 4 +- > drivers/firmware/efi/libstub/Makefile | 2 + > drivers/misc/lkdtm/Makefile | 1 + > include/asm-generic/vmlinux.lds.h | 11 +- > include/linux/init.h | 79 +++++++- > include/linux/pci.h | 19 +- > kernel/trace/Kconfig | 16 ++ > scripts/Makefile.build | 50 ++++- > scripts/Makefile.lib | 6 +- > scripts/Makefile.modfinal | 9 +- > scripts/Makefile.modpost | 25 ++- > scripts/generate_initcall_order.pl | 270 ++++++++++++++++++++++++++ > scripts/link-vmlinux.sh | 70 ++++++- > scripts/mod/Makefile | 1 + > scripts/mod/modpost.c | 16 +- > scripts/mod/modpost.h | 9 + > scripts/mod/sumversion.c | 6 +- > scripts/module.lds.S | 24 +++ > 23 files changed, 677 insertions(+), 68 deletions(-) > create mode 100755 scripts/generate_initcall_order.pl > > > base-commit: 0fa8ee0d9ab95c9350b8b84574824d9a384a9f7d > -- > 2.29.2.299.gdc1121823c-goog > -- Thanks, ~Nick Desaulniers
