mirror of
https://github.com/torvalds/linux.git
synced 2024-11-22 12:11:40 +00:00
c1f9e14e3b
Add brief text about existence of helper functions, with details to go in separate psABI text. Note that text about runtime functions (kfuncs) is part of a separate patch, not this one. Signed-off-by: Dave Thaler <dthaler@microsoft.com> Link: https://lore.kernel.org/r/20230308205303.1308-1-dthaler1968@googlemail.com Signed-off-by: Alexei Starovoitov <ast@kernel.org>
37 lines
1.2 KiB
ReStructuredText
37 lines
1.2 KiB
ReStructuredText
.. 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``.
|