[PATCH v8 00/16] Add support for Clang LTO

[Fangrui Song]

On 2020-12-08, 'Sami Tolvanen' via Clang Built Linux wrote:
>On Tue, Dec 8, 2020 at 4:15 AM Arnd Bergmann <arnd at kernel.org> wrote:
>>
>> On Tue, Dec 1, 2020 at 10:37 PM 'Sami Tolvanen' via Clang Built Linux
>> <clang-built-linux at googlegroups.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 arm64 support depends on Will's memory ordering patches
>> > [1]. I will post x86_64 patches separately after we have fixed the
>> > remaining objtool warnings [2][3].
>> >
>> > [1] https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/log/?h=for-next/lto
>> > [2] https://lore.kernel.org/lkml/20201120040424.a3wctajzft4ufoiw@treble/
>> > [3] https://git.kernel.org/pub/scm/linux/kernel/git/jpoimboe/linux.git/log/?h=objtool-vmlinux
>> >
>> > You can also pull this series from
>> >
>> >   https://github.com/samitolvanen/linux.git lto-v8
>>
>> I've tried pull this into my randconfig test tree to give it a spin.
>
>Great, thank you for testing this!
>
>> So far I have
>> not managed to get a working build out of it, the main problem so far being
>> that it is really slow to build because the link stage only uses one CPU.
>> These are the other issues I've seen so far:

ld.lld ThinLTO uses the number of (physical cores enabled by affinity) by default.

>You may want to limit your testing only to ThinLTO at first, because
>full LTO is going to be extremely slow with larger configs, especially
>when building arm64 kernels.
>
>> - one build seems to take even longer to link. It's currently at 35GB RAM
>>   usage and 40 minutes into the final link, but I'm worried it might
>> not complete
>>   before it runs out of memory.  I only have 128GB installed, and google-chrome
>>   uses another 30GB of that, and I'm also doing some other builds in parallel.
>>   Is there a minimum recommended amount of memory for doing LTO builds?
>
>When building arm64 defconfig, the maximum memory usage I measured
>with ThinLTO was 3.5 GB, and with full LTO 20.3 GB. I haven't measured
>larger configurations, but I believe LLD can easily consume 3-4x that
>much with full LTO allyesconfig.
>
>> - One build failed with
>>  ld.lld -EL -maarch64elf -mllvm -import-instr-limit=5 -r -o vmlinux.o
>> -T .tmp_initcalls.lds --whole-archive arch/arm64/kernel/head.o
>> init/built-in.a usr/built-in.a arch/arm64/built-in.a kernel/built-in.a
>> certs/built-in.a mm/built-in.a fs/built-in.a ipc/built-in.a
>> security/built-in.a crypto/built-in.a block/built-in.a
>> arch/arm64/lib/built-in.a lib/built-in.a drivers/built-in.a
>> sound/built-in.a net/built-in.a virt/built-in.a --no-whole-archive
>> --start-group arch/arm64/lib/lib.a lib/lib.a
>> ./drivers/firmware/efi/libstub/lib.a --end-group
>>   "ld.lld: error: arch/arm64/kernel/head.o: invalid symbol index"
>>   after about 30 minutes
>
>That's interesting. Did you use LLVM_IAS=1?

May be worth checking which relocation or (SHT_GROUP section's sh_info) in arch/arm64/kernel/head.o is incorrect.

>> - CONFIG_CPU_BIG_ENDIAN doesn't seem to work with lld, and LTO
>>   doesn't work with ld.bfd.
>>   I've added a CPU_LITTLE_ENDIAN dependency to
>>   ARCH_SUPPORTS_LTO_CLANG{,THIN}
>
>Ah, good point. I'll fix this in v9.

Full/Thin LTO should work with GNU ld and gold with LLVMgold.so built from
llvm-project (https://llvm.org/docs/GoldPlugin.html ). You'll need to make sure
that LLVMgold.so is newer than clang. (Newer clang may introduce bitcode
attributes which are unrecognizable by older LLVMgold.so/ld.lld)

>[...]
>> Not sure if these are all known issues. If there is one you'd like me try
>> take a closer look at for finding which config options break it, I can try
>
>No, none of these are known issues. I would be happy to take a closer
>look if you can share configs that reproduce these.
>
>Sami
>
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