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bpf, docs: Move Clang notes to a separate file

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Move Clang notes to a separate file.

Signed-off-by: Dave Thaler <dthaler@microsoft.com>
Link: https://lore.kernel.org/r/20220927185958.14995-3-dthaler1968@googlemail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Dave Thaler 2022-09-27 18:59:46 +00:00 committed by Alexei Starovoitov
parent 9a0bf21337
commit 6c7aaffb24
2 changed files with 24 additions and 6 deletions
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24
Documentation/bpf/clang-notes.rst Normal file
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@ -0,0 +1,24 @@
.. contents::
.. sectnum::
==========================
Clang implementation notes
==========================
This document provides more details specific to the Clang/LLVM implementation of the eBPF instruction set.
Versions
========
Clang defined "CPU" versions, where a CPU version of 3 corresponds to the current eBPF ISA.
Clang can select the eBPF ISA version using ``-mcpu=v3`` for example to select version 3.
Atomic operations
=================
Clang can generate atomic instructions by default when ``-mcpu=v3`` is
enabled. If a lower version for ``-mcpu`` is set, the only atomic instruction
Clang can generate is ``BPF_ADD`` *without* ``BPF_FETCH``. If you need to enable
the atomics features, while keeping a lower ``-mcpu`` version, you can use
``-Xclang -target-feature -Xclang +alu32``.

6
Documentation/bpf/instruction-set.rst
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@ -303,12 +303,6 @@ The ``BPF_CMPXCHG`` operation atomically compares the value addressed by
value that was at ``dst_reg + off`` before the operation is zero-extended
and loaded back to ``R0``.
Clang can generate atomic instructions by default when ``-mcpu=v3`` is
enabled. If a lower version for ``-mcpu`` is set, the only atomic instruction
Clang can generate is ``BPF_ADD`` *without* ``BPF_FETCH``. If you need to enable
the atomics features, while keeping a lower ``-mcpu`` version, you can use
``-Xclang -target-feature -Xclang +alu32``.
64-bit immediate instructions
-----------------------------