On Tue, Dec 8, 2020 at 4:15 AM Arnd Bergmann <arnd@xxxxxxxxxx> wrote: > > On Tue, Dec 1, 2020 at 10:37 PM 'Sami Tolvanen' via Clang Built Linux > <clang-built-linux@xxxxxxxxxxxxxxxx> 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: 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? > - 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. [...] > 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
