bpf: Document new atomic instructions

Document new atomic instructions.
Signed-off-by: Brendan Jackman <jackmanb@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210114181751.768687-12-jackmanb@google.com

Documentation/networking/filter.rst

@@ -1053,8 +1053,39 @@ encoding.
    .imm = BPF_ADD, .code = BPF_ATOMIC | BPF_W  | BPF_STX: lock xadd *(u32 *)(dst_reg + off16) += src_reg
    .imm = BPF_ADD, .code = BPF_ATOMIC | BPF_DW | BPF_STX: lock xadd *(u64 *)(dst_reg + off16) += src_reg
 
+The basic atomic operations supported are:
+
+    BPF_ADD
+    BPF_AND
+    BPF_OR
+    BPF_XOR
+
+Each having equivalent semantics with the ``BPF_ADD`` example, that is: the
+memory location addresed by ``dst_reg + off`` is atomically modified, with
+``src_reg`` as the other operand. If the ``BPF_FETCH`` flag is set in the
+immediate, then these operations also overwrite ``src_reg`` with the
+value that was in memory before it was modified.
+
+The more special operations are:
+
+    BPF_XCHG
+
+This atomically exchanges ``src_reg`` with the value addressed by ``dst_reg +
+off``.
+
+    BPF_CMPXCHG
+
+This atomically compares the value addressed by ``dst_reg + off`` with
+``R0``. If they match it is replaced with ``src_reg``, The value that was there
+before is loaded back to ``R0``.
+
 Note that 1 and 2 byte atomic operations are not supported.
 
+Except ``BPF_ADD`` _without_ ``BPF_FETCH`` (for legacy reasons), all 4 byte
+atomic operations require alu32 mode. Clang enables this mode by default in
+architecture v3 (``-mcpu=v3``). For older versions it can be enabled with
+``-Xclang -target-feature -Xclang +alu32``.
+
 You may encounter BPF_XADD - this is a legacy name for BPF_ATOMIC, referring to
 the exclusive-add operation encoded when the immediate field is zero.