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2024-02-20 ea08eeccae9297f7aabd2ef7f0c2517ac4549acc

 kernel/samples/bpf/README.rst
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 Note that LLVM's tool 'llc' must support target 'bpf', list version
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 and supported targets with command: ``llc --version``
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+Clean and configuration
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+-----------------------
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+
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+It can be needed to clean tools, samples or kernel before trying new arch or
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+after some changes (on demand)::
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+
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+ make -C tools clean
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+ make -C samples/bpf clean
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+ make clean
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+
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+Configure kernel, defconfig for instance::
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+
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+ make defconfig
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+
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 Kernel headers
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 --------------
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 For building the BPF samples, issue the below command from the kernel
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 top level directory::
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- make samples/bpf/
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-
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-Do notice the "/" slash after the directory name.
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+ make M=samples/bpf
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 It is also possible to call make from this directory.  This will just
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-hide the the invocation of make as above with the appended "/".
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+hide the invocation of make as above.
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 Manually compiling LLVM with 'bpf' support
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 ------------------------------------------
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 It is also possible to point make to the newly compiled 'llc' or
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 'clang' command via redefining LLC or CLANG on the make command line::
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- make samples/bpf/ LLC=~/git/llvm/build/bin/llc CLANG=~/git/llvm/build/bin/clang
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+ make M=samples/bpf LLC=~/git/llvm/build/bin/llc CLANG=~/git/llvm/build/bin/clang
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 Cross compiling samples
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 -----------------------
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 In order to cross-compile, say for arm64 targets, export CROSS_COMPILE and ARCH
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-environment variables before calling make. This will direct make to build
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-samples for the cross target.
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+environment variables before calling make. But do this before clean,
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+cofiguration and header install steps described above. This will direct make to
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+build samples for the cross target::
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-export ARCH=arm64
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-export CROSS_COMPILE="aarch64-linux-gnu-"
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-make samples/bpf/ LLC=~/git/llvm/build/bin/llc CLANG=~/git/llvm/build/bin/clang
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+ export ARCH=arm64
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+ export CROSS_COMPILE="aarch64-linux-gnu-"
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+
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+Headers can be also installed on RFS of target board if need to keep them in
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+sync (not necessarily and it creates a local "usr/include" directory also)::
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+
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+ make INSTALL_HDR_PATH=~/some_sysroot/usr headers_install
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+
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+Pointing LLC and CLANG is not necessarily if it's installed on HOST and have
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+in its targets appropriate arm64 arch (usually it has several arches).
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+Build samples::
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+
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+ make M=samples/bpf
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+
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+Or build samples with SYSROOT if some header or library is absent in toolchain,
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+say libelf, providing address to file system containing headers and libs,
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+can be RFS of target board::
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+
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+ make M=samples/bpf SYSROOT=~/some_sysroot