Update ISA documentation

isa/README.md

@@ -8,10 +8,14 @@ Some documentation links include:
 * https://www.kernel.org/doc/Documentation/networking/filter.txt
 * https://pchaigno.github.io/bpf/2021/10/20/ebpf-instruction-sets.html
 * https://www.kernel.org/doc/html/latest/bpf/bpf_design_QA.html#instruction-level-questions
+* https://lore.kernel.org/bpf/[email protected]/
+* https://github.com/iovisor/bpf-docs/pull/26/files
 
 Some implementation links include:
 
 * https://github.com/iovisor/ubpf/blob/master/vm/ebpf.h
 * https://github.com/generic-ebpf/generic-ebpf/blob/dev/sys/sys/ebpf_vm_isa.h
 * https://github.com/vbpf/ebpf-verifier/blob/main/src/ebpf_vm_isa.hpp
 * https://elixir.bootlin.com/linux/latest/source/include/uapi/linux/bpf.h
+* https://lore.kernel.org/bpf/[email protected]/
+* https://github.com/iovisor/bpf-docs/pull/26/files

isa/kernel.org/clang-notes.rst

@@ -0,0 +1,35 @@
+.. 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.
+
+Invalid instructions
+====================
+
+Clang will generate the invalid ``BPF_CALL | BPF_X | BPF_JMP`` (0x8d) instruction if ``-O0`` is used.
+
+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``.