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    Subject[PATCH bpf-next v3 14/14] bpf: Document new atomic instructions
    From
    Change-Id: Ic70fe9e3cb4403df4eb3be2ea5ae5af53156559e
    Signed-off-by: Brendan Jackman <jackmanb@google.com>
    ---
    Documentation/networking/filter.rst | 26 ++++++++++++++++++++++++++
    1 file changed, 26 insertions(+)

    diff --git a/Documentation/networking/filter.rst b/Documentation/networking/filter.rst
    index 1583d59d806d..26d508a5e038 100644
    --- a/Documentation/networking/filter.rst
    +++ b/Documentation/networking/filter.rst
    @@ -1053,6 +1053,32 @@ 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 (from architecture v4 onwards) 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.

    You may encounter BPF_XADD - this is a legacy name for BPF_ATOMIC, referring to
    --
    2.29.2.454.gaff20da3a2-goog
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