On Fri, Nov 20, 2020 at 11:29:51AM +0100, Ard Biesheuvel wrote: > On Thu, 19 Nov 2020 at 00:42, Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote: > > > > 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. > > > > When you say 'virtualized" do you mean QEMU on x86? Or actual > virtualization on an AArch64 KVM host? > > The distinction is important here, given the potential impact of LTO > on things that QEMU simply does not model when it runs in TCG mode on > a foreign host architecture. I have booted this series on my Raspberry Pi 4 (ARCH=arm64 defconfig). $ uname -r 5.10.0-rc4-00108-g830200082c74 $ zgrep LTO /proc/config.gz CONFIG_LTO=y CONFIG_ARCH_SUPPORTS_LTO_CLANG=y CONFIG_ARCH_SUPPORTS_THINLTO=y CONFIG_THINLTO=y # CONFIG_LTO_NONE is not set CONFIG_LTO_CLANG=y # CONFIG_HID_WALTOP is not set and I have taken that same kernel and booted it under QEMU with '-enable-kvm' without any visible issues. I have tested four combinations: clang 12 @ f9f0a4046e11c2b4c130640f343e3b2b5db08c1: * CONFIG_THINLTO=y * CONFIG_THINLTO=n clang 11.0.0 * CONFIG_THINLTO=y * CONFIG_THINLTO=n Tested-by: Nathan Chancellor <natechancellor@xxxxxxxxx> Cheers, Nathan
