Adding upstream version 6.6.15.upstream/6.6.15

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 36 insertions, 0 deletions

diff --git a/Documentation/bpf/clang-notes.rst b/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.
+
+Arithmetic instructions
+=======================
+
+For CPU versions prior to 3, Clang v7.0 and later can enable ``BPF_ALU`` support with
+``-Xclang -target-feature -Xclang +alu32``.  In CPU version 3, support is automatically included.
+
+Jump instructions
+=================
+
+If ``-O0`` is used, Clang will generate the ``BPF_CALL | BPF_X | BPF_JMP`` (0x8d)
+instruction, which is not supported by the Linux kernel verifier.
+
+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``.