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

[Tiezhu Yang]

On 01/20/2021 10:43 AM, Nick Desaulniers wrote:
> On Mon, Jan 18, 2021 at 11:56 PM Tiezhu Yang <yangtiezhu@xxxxxxxxxxx> 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 out of date.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@xxxxxxxxxxx>
> > Acked-by: Yonghong Song <yhs@xxxxxx>
> > ---
> >
> > 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?

The default value includes BPF.

>   I frown upon disabling other backends.

I think the initial aim of the build procedure in samples/bpf/README.rst [1]
is a simple and quick start guide:

[ To generate a smaller llc binary one can use:

 -DLLVM_TARGETS_TO_BUILD="BPF" ]

[1] https://github.com/torvalds/linux/blob/master/samples/bpf/README.rst

> > - $ 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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