From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oi1-f195.google.com ([209.85.167.195]:33521 "EHLO mail-oi1-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726962AbgBYEQr (ORCPT ); Mon, 24 Feb 2020 23:16:47 -0500 Date: Mon, 24 Feb 2020 21:16:43 -0700 From: Nathan Chancellor Subject: Re: [PATCH] Documentation/llvm: add documentation on building w/ Clang/LLVM Message-ID: <20200225041643.GA17425@ubuntu-m2-xlarge-x86> References: <20200224174129.2664-1-ndesaulniers@google.com> <202002242003.870E5F80@keescook> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <202002242003.870E5F80@keescook> Sender: linux-kbuild-owner@vger.kernel.org List-ID: To: Kees Cook Cc: Nick Desaulniers , corbet@lwn.net, masahiroy@kernel.org, Michal Marek , linux-kbuild@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com On Mon, Feb 24, 2020 at 08:08:26PM -0800, Kees Cook wrote: > On Mon, Feb 24, 2020 at 09:41:28AM -0800, Nick Desaulniers wrote: > > Added to kbuild documentation. Provides more official info on building > > kernels with Clang and LLVM than our wiki. > > > > Suggested-by: Kees Cook > > Reviewed-by: Nathan Chancellor > > Signed-off-by: Nick Desaulniers > > --- > > Documentation/kbuild/index.rst | 1 + > > Documentation/kbuild/llvm.rst | 80 ++++++++++++++++++++++++++++++++++ > > 2 files changed, 81 insertions(+) > > create mode 100644 Documentation/kbuild/llvm.rst > > > > diff --git a/Documentation/kbuild/index.rst b/Documentation/kbuild/index.rst > > index 0f144fad99a6..3882bd5f7728 100644 > > --- a/Documentation/kbuild/index.rst > > +++ b/Documentation/kbuild/index.rst > > @@ -19,6 +19,7 @@ Kernel Build System > > > > issues > > reproducible-builds > > + llvm > > > > .. only:: subproject and html > > > > diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst > > new file mode 100644 > > index 000000000000..68ae022aebc0 > > --- /dev/null > > +++ b/Documentation/kbuild/llvm.rst > > @@ -0,0 +1,80 @@ > > +============================== > > +Building Linux with Clang/LLVM > > +============================== > > + > > +This document covers how to build the Linux kernel with Clang and LLVM > > +utilities. > > + > > +About > > +----- > > + > > +The Linux kernel has always traditionally been compiled with GNU toolchains > > +such as GCC and binutils. On going work has allowed for `Clang > > +`_ and `LLVM `_ utilities to be > > +used as viable substitutes. Distributions such as `Android > > +`_, `ChromeOS > > +`_, and `OpenMandriva > > +`_ use Clang built kernels. `LLVM is a > > +collection of toolchain components implemented in terms of C++ objects > > +`_. Clang is a front-end to LLVM that > > +supports C and the GNU C extensions required by the kernel, and is pronounced > > +"klang," not "see-lang." > > + > > +Clang > > +----- > > + > > +The compiler used can be swapped out via `CC=` command line argument to `make`. > > +`CC=` should be set when selecting a config and during a build. > > + > > + make CC=clang defconfig > > + > > + make CC=clang > > + > > +Cross Compiling > > +--------------- > > + > > +A single Clang compiler binary will typically contain all supported backends, > > +which can help simplify cross compiling. > > + > > + ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make CC=clang > > + > > +`CROSS_COMPILE` is not used to suffix the Clang compiler binary, instead > > s/suffix/prefix/ > > > +`CROSS_COMPILE` is used to set a command line flag: `--target `. For > > +example: > > + > > + clang --target aarch64-linux-gnu foo.c > > + > > +LLVM Utilities > > +-------------- > > + > > +LLVM has substitutes for GNU binutils utilities. These can be invoked as > > +additional parameters to `make`. > > + > > + make CC=clang AS=clang LD=ld.lld AR=llvm-ar NM=llvm-nm STRIP=llvm-strip \\ > > + OBJCOPY=llvm-objcopy OBJDUMP=llvm-objdump OBJSIZE=llvm-objsize \\ > > + READELF=llvm-readelf HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar \\ > > + HOSTLD=ld.lld > > + > > +Getting Help > > +------------ > > + > > +- `Website `_ > > +- `Mailing List `_: > > +- `Issue Tracker `_ > > +- IRC: #clangbuiltlinux on chat.freenode.net > > +- `Telegram `_: @ClangBuiltLinux > > +- `Wiki `_ > > +- `Beginner Bugs `_ > > + > > +Getting LLVM > > +------------- > > + > > +- http://releases.llvm.org/download.html > > +- https://github.com/llvm/llvm-project > > +- https://llvm.org/docs/GettingStarted.html > > +- https://llvm.org/docs/CMake.html > > +- https://apt.llvm.org/ > > +- https://www.archlinux.org/packages/extra/x86_64/llvm/ > > +- https://github.com/ClangBuiltLinux/tc-build > > +- https://github.com/ClangBuiltLinux/linux/wiki/Building-Clang-from-source > > +- https://android.googlesource.com/platform/prebuilts/clang/host/linux-x86/ > > Should this also include an update to Documentation/process/changes.rst > with the minimum version required? (I would expect this to be "9" for Clang, > and "11" for ld.lld.) I think the clang one should be added in a separate patch that solidifies that in include/linux/compiler-clang.h with a CLANG_VERSION macro and version check, like in include/linux/compiler-gcc.h. ld.lld's minimum version should also be 9, what is the blocking issue that makes it 11? > Otherwise, yes, with Randy and Masahiro's suggestions, please consider it: > > Reviewed-by: Kees Cook > > -- > Kees Cook Cheers, Nathan