1 Clang implementation notes¶
This document provides more details specific to the Clang/LLVM implementation of the eBPF instruction set.
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.
1.2 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.
1.3 Jump instructions¶
-O0 is used, Clang will generate the
BPF_CALL | BPF_X | BPF_JMP (0x8d)
instruction, which is not supported by the Linux kernel verifier.
1.4 Atomic operations¶
Clang can generate atomic instructions by default when
enabled. If a lower version for
-mcpu is set, the only atomic instruction
Clang can generate is
BPF_FETCH. If you need to enable
the atomics features, while keeping a lower
-mcpu version, you can use
-Xclang -target-feature -Xclang +alu32.