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Date:   Tue, 12 Jan 2021 15:42:35 +0000
From:   Brendan Jackman <>
Cc:     Alexei Starovoitov <>,
        Daniel Borkmann <>,
        Andrii Nakryiko <>,
        KP Singh <>,
        Florent Revest <>,,
        "Björn Töpel" <>,
        John Fastabend <>,
        Yonghong Song <>,
        Brendan Jackman <>
Subject: [PATCH bpf-next v6 11/11] bpf: Document new atomic instructions

Document new atomic instructions.

Acked-by: Yonghong Song <>
Signed-off-by: Brendan Jackman <>
 Documentation/networking/filter.rst | 31 +++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/Documentation/networking/filter.rst b/Documentation/networking/filter.rst
index 1583d59d806d..f6d8f90e9a56 100644
--- a/Documentation/networking/filter.rst
+++ b/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:
+This atomically exchanges ``src_reg`` with the value addressed by ``dst_reg +
+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.

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