[linux-block.git] / Documentation / bpf / clang-notes.rst

.. contents::
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==========================
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``.
Commit	Line	Data
6c7aaffb DT	1	.. contents::
	2	.. sectnum::
	3
	4	==========================
	5	Clang implementation notes
	6	==========================
	7
	8	This document provides more details specific to the Clang/LLVM implementation of the eBPF instruction set.
	9
	10	Versions
	11	========
	12
	13	Clang defined "CPU" versions, where a CPU version of 3 corresponds to the current eBPF ISA.
	14
	15	Clang can select the eBPF ISA version using ``-mcpu=v3`` for example to select version 3.
	16
ee159bdb DT	17	Arithmetic instructions
	18	=======================
	19
	20	For CPU versions prior to 3, Clang v7.0 and later can enable ``BPF_ALU`` support with
	21	``-Xclang -target-feature -Xclang +alu32``. In CPU version 3, support is automatically included.
	22
c1f9e14e DT	23	Jump instructions
	24	=================
	25
	26	If ``-O0`` is used, Clang will generate the ``BPF_CALL \| BPF_X \| BPF_JMP`` (0x8d)
	27	instruction, which is not supported by the Linux kernel verifier.
	28
6c7aaffb DT	29	Atomic operations
	30	=================
	31
	32	Clang can generate atomic instructions by default when ``-mcpu=v3`` is
	33	enabled. If a lower version for ``-mcpu`` is set, the only atomic instruction
	34	Clang can generate is ``BPF_ADD`` without ``BPF_FETCH``. If you need to enable
	35	the atomics features, while keeping a lower ``-mcpu`` version, you can use
	36	``-Xclang -target-feature -Xclang +alu32``.