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    SubjectRe: [PATCH v7 00/17] Add support for Clang LTO
    On Wed, Nov 18, 2020 at 2:07 PM Sami Tolvanen <samitolvanen@google.com> 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@google.com>

    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

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