linux-kernel - Re: [PATCH bpf-next v2] samples/bpf: Update README.rst and Makefile for manually compiling LLVM and clang

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Date:   Tue, 19 Jan 2021 19:06:11 -0800
From:   Yonghong Song <yhs@...com>
To:     Tiezhu Yang <yangtiezhu@...ngson.cn>,
        Fangrui Song <maskray@...gle.com>
CC:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        <netdev@...r.kernel.org>, <bpf@...r.kernel.org>,
        <clang-built-linux@...glegroups.com>,
        <linux-kernel@...r.kernel.org>, Xuefeng Li <lixuefeng@...ngson.cn>
Subject: Re: [PATCH bpf-next v2] samples/bpf: Update README.rst and Makefile
 for manually compiling LLVM and clang



On 1/19/21 6:32 PM, Tiezhu Yang wrote:
> On 01/20/2021 05:58 AM, Fangrui Song wrote:
>> On 2021-01-19, Tiezhu Yang 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 out of date.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 
>>>
>>> Signed-off-by: Tiezhu Yang <yangtiezhu@...ngson.cn>
>>> Acked-by: Yonghong Song <yhs@...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"
>>> - $ make -j $(getconf _NPROCESSORS_ONLN)
>>> + $ git clone https://github.com/llvm/llvm-project.git
>>> + $ mkdir -p llvm-project/llvm/build/install
>>
>> llvm-project/llvm/build/install is not used.
> 
> Yes, just mkdir -p llvm-project/llvm/build is OK.
> 
>>
>>> + $ cd llvm-project/llvm/build
>>> + $ cmake .. -G "Ninja" -DLLVM_TARGETS_TO_BUILD="BPF;X86" \
>>> +            -DLLVM_ENABLE_PROJECTS="clang"    \
>>> +            -DBUILD_SHARED_LIBS=OFF           \
>>
>> -DBUILD_SHARED_LIBS=OFF is the default. It can be omitted.
> 
> I search the related doc [1] [2], remove this option is OK for me.
> 
> BUILD_SHARED_LIBS:BOOL
> 
>      Flag indicating if each LLVM component (e.g. Support) is built as a 
> shared library (ON) or as a static library (OFF). Its default value is OFF.
> 
> [1] 
> https://www.llvm.org/docs/CMake.html 
> [2] 
> https://cmake.org/cmake/help/latest/variable/BUILD_SHARED_LIBS.html 
> 
>>
>>> + -DCMAKE_BUILD_TYPE=Release        \
>>> +            -DLLVM_BUILD_RUNTIME=OFF
>>
>> -DLLVM_BUILD_RUNTIME=OFF can be omitted if none of
>> compiler-rt/libc++/libc++abi is built.
> 
> I am not very sure about it because the default value of
> LLVM_BUILD_RUNTIME is ON? [3]
> 
> option(LLVM_BUILD_RUNTIME
> "Build the LLVM runtime libraries." ON)
> 
> [3] https://github.com/llvm/llvm-project/blob/main/llvm/CMakeLists.txt
> 
> If anyone has any more suggestions, please let me know.
> I will send v3 after waiting for other feedback.
> 
> By the way, Documentation/bpf/bpf_devel_QA.rst maybe need a separate
> patch to remove some cmake options?

Please submit updated this patch and Documentation/bpf/bpf_devel_QA.rst
together. This way, it is easy to cross check. Thanks.

> 
> Thanks,
> Tiezhu
> 
[...]