From: Dave Thaler <dthaler@xxxxxxxxxxxxx> Signed-off-by: Dave Thaler <dthaler@xxxxxxxxxxxxx> --- Documentation/bpf/clang-notes.rst | 24 ++++++++++++++++++++++++ Documentation/bpf/instruction-set.rst | 6 ------ 2 files changed, 24 insertions(+), 6 deletions(-) create mode 100644 Documentation/bpf/clang-notes.rst diff --git a/Documentation/bpf/clang-notes.rst b/Documentation/bpf/clang-notes.rst new file mode 100644 index 000000000..b15179cb5 --- /dev/null +++ b/Documentation/bpf/clang-notes.rst @@ -0,0 +1,24 @@ +.. 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``. diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst index 1735b91ec..541483118 100644 --- a/Documentation/bpf/instruction-set.rst +++ b/Documentation/bpf/instruction-set.rst @@ -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 ----------------------------- -- 2.33.4
