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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Tue, 19 Jan 2021 18:43:36 -0800
Message-ID: <CAKwvOdkXGx-WogH0o5iuNnEe07sqRfxMpOg5fEEnTWcOfBrbAQ@mail.gmail.com>
Subject: Re: [PATCH bpf-next v2] samples/bpf: Update README.rst and Makefile
 for manually compiling LLVM and clang
To:     Tiezhu Yang <yangtiezhu@loongson.cn>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Nathan Chancellor <natechancellor@gmail.com>,
        Network Development <netdev@vger.kernel.org>,
        bpf <bpf@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Xuefeng Li <lixuefeng@loongson.cn>
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On Mon, Jan 18, 2021 at 11:56 PM Tiezhu Yang <yangtiezhu@loongson.cn> wrote:
>
> The current llvm/clang build procedure in samples/bpf/README.rst is
> out of date. See below that the links are not accessible any more.
>
> $ git clone http://llvm.org/git/llvm.git
> Cloning into 'llvm'...
> fatal: unable to access 'http://llvm.org/git/llvm.git/': Maximum (20) redirects followed
> $ git clone --depth 1 http://llvm.org/git/clang.git
> Cloning into 'clang'...
> fatal: unable to access 'http://llvm.org/git/clang.git/': Maximum (20) redirects followed
>
> The llvm community has adopted new ways to build the compiler. There are
> different ways to build llvm/clang, the Clang Getting Started page [1] has
> one way. As Yonghong said, it is better to just copy the build procedure
> in Documentation/bpf/bpf_devel_QA.rst to keep consistent.
>
> I verified the procedure and it is proved to be feasible, so we should
> update README.rst to reflect the reality. At the same time, update the
> related comment in Makefile.
>
> [1] https://clang.llvm.org/get_started.html

There's also https://www.kernel.org/doc/html/latest/kbuild/llvm.html#getting-llvm
(could cross link in rst/sphinx).

>
> Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
> Acked-by: Yonghong Song <yhs@fb.com>
> ---
>
> v2: Update the commit message suggested by Yonghong,
>     thank you very much.
>
>  samples/bpf/Makefile   |  2 +-
>  samples/bpf/README.rst | 17 ++++++++++-------
>  2 files changed, 11 insertions(+), 8 deletions(-)
>
> diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
> index 26fc96c..d061446 100644
> --- a/samples/bpf/Makefile
> +++ b/samples/bpf/Makefile
> @@ -208,7 +208,7 @@ TPROGLDLIBS_xdpsock         += -pthread -lcap
>  TPROGLDLIBS_xsk_fwd            += -pthread
>
>  # Allows pointing LLC/CLANG to a LLVM backend with bpf support, redefine on cmdline:
> -#  make M=samples/bpf/ LLC=~/git/llvm/build/bin/llc CLANG=~/git/llvm/build/bin/clang
> +# make M=samples/bpf LLC=~/git/llvm-project/llvm/build/bin/llc CLANG=~/git/llvm-project/llvm/build/bin/clang
>  LLC ?= llc
>  CLANG ?= clang
>  OPT ?= opt
> diff --git a/samples/bpf/README.rst b/samples/bpf/README.rst
> index dd34b2d..d1be438 100644
> --- a/samples/bpf/README.rst
> +++ b/samples/bpf/README.rst
> @@ -65,17 +65,20 @@ To generate a smaller llc binary one can use::
>  Quick sniplet for manually compiling LLVM and clang
>  (build dependencies are cmake and gcc-c++)::
>
> - $ git clone http://llvm.org/git/llvm.git
> - $ cd llvm/tools
> - $ git clone --depth 1 http://llvm.org/git/clang.git
> - $ cd ..; mkdir build; cd build
> - $ cmake .. -DLLVM_TARGETS_TO_BUILD="BPF;X86"

Is the BPF target not yet on by default?  I frown upon disabling other backends.

> - $ make -j $(getconf _NPROCESSORS_ONLN)
> + $ git clone https://github.com/llvm/llvm-project.git
> + $ mkdir -p llvm-project/llvm/build/install
> + $ cd llvm-project/llvm/build
> + $ cmake .. -G "Ninja" -DLLVM_TARGETS_TO_BUILD="BPF;X86" \
> +            -DLLVM_ENABLE_PROJECTS="clang"    \
> +            -DBUILD_SHARED_LIBS=OFF           \
> +            -DCMAKE_BUILD_TYPE=Release        \
> +            -DLLVM_BUILD_RUNTIME=OFF
> + $ ninja
>
>  It is also possible to point make to the newly compiled 'llc' or
>  'clang' command via redefining LLC or CLANG on the make command line::
>
> - make M=samples/bpf LLC=~/git/llvm/build/bin/llc CLANG=~/git/llvm/build/bin/clang
> + make M=samples/bpf LLC=~/git/llvm-project/llvm/build/bin/llc CLANG=~/git/llvm-project/llvm/build/bin/clang
>
>  Cross compiling samples
>  -----------------------
> --
> 2.1.0
>
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-- 
Thanks,
~Nick Desaulniers