Commit 6c7aaffb authored by Dave Thaler's avatar Dave Thaler Committed by Alexei Starovoitov
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bpf, docs: Move Clang notes to a separate file 



Move Clang notes to a separate file.

Signed-off-by: default avatarDave Thaler <dthaler@microsoft.com>
Link: https://lore.kernel.org/r/20220927185958.14995-3-dthaler1968@googlemail.com


Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
parent 9a0bf213

Documentation/bpf/clang-notes.rst

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.. 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``.

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

-----------------------------