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Subject[PATCH v7 00/17] Add support for Clang LTO
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].


You can also pull this series from lto-v7

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

Changes in v6:

- Added the missing --mcount flag to patch 5.

- Dropped the arm64 patches from this series and will repost them

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

- 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,
and moved the contents of to

- 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 to use jobserver-exec to control the
number of jobs started by

- 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/ | 11 +-
include/linux/init.h | 79 +++++++-
include/linux/pci.h | 19 +-
kernel/trace/Kconfig | 16 ++
scripts/ | 50 ++++-
scripts/Makefile.lib | 6 +-
scripts/Makefile.modfinal | 9 +-
scripts/Makefile.modpost | 25 ++-
scripts/ | 270 ++++++++++++++++++++++++++
scripts/ | 70 ++++++-
scripts/mod/Makefile | 1 +
scripts/mod/modpost.c | 16 +-
scripts/mod/modpost.h | 9 +
scripts/mod/sumversion.c | 6 +-
scripts/ | 24 +++
23 files changed, 677 insertions(+), 68 deletions(-)
create mode 100755 scripts/

base-commit: 0fa8ee0d9ab95c9350b8b84574824d9a384a9f7d

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