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Add Clang note about BPF_ALU. Signed-off-by: Dave Thaler <dthaler@microsoft.com> Link: https://lore.kernel.org/r/20220927185958.14995-4-dthaler1968@googlemail.com Signed-off-by: Alexei Starovoitov <ast@kernel.org>
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.. contents::
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==========================
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Clang implementation notes
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==========================
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This document provides more details specific to the Clang/LLVM implementation of the eBPF instruction set.
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Versions
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========
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Clang defined "CPU" versions, where a CPU version of 3 corresponds to the current eBPF ISA.
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Clang can select the eBPF ISA version using ``-mcpu=v3`` for example to select version 3.
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Arithmetic instructions
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=======================
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For CPU versions prior to 3, Clang v7.0 and later can enable ``BPF_ALU`` support with
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``-Xclang -target-feature -Xclang +alu32``. In CPU version 3, support is automatically included.
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Atomic operations
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=================
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Clang can generate atomic instructions by default when ``-mcpu=v3`` is
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enabled. If a lower version for ``-mcpu`` is set, the only atomic instruction
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Clang can generate is ``BPF_ADD`` *without* ``BPF_FETCH``. If you need to enable
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the atomics features, while keeping a lower ``-mcpu`` version, you can use
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``-Xclang -target-feature -Xclang +alu32``.
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