linux-kernel - [PATCH v2 bpf-next 13/13] bpf: Document new atomic instructions

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Message-Id: <20201127175738.1085417-14-jackmanb@google.com>
Date:   Fri, 27 Nov 2020 17:57:38 +0000
From:   Brendan Jackman <jackmanb@...gle.com>
To:     bpf@...r.kernel.org
Cc:     Alexei Starovoitov <ast@...nel.org>, Yonghong Song <yhs@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        KP Singh <kpsingh@...omium.org>,
        Florent Revest <revest@...omium.org>,
        linux-kernel@...r.kernel.org, Jann Horn <jannh@...gle.com>,
        Brendan Jackman <jackmanb@...gle.com>
Subject: [PATCH v2 bpf-next 13/13] bpf: Document new atomic instructions

Signed-off-by: Brendan Jackman <jackmanb@...gle.com>
---
 Documentation/networking/filter.rst | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/Documentation/networking/filter.rst b/Documentation/networking/filter.rst
index 1583d59d806d..c86091b8cb0e 100644
--- a/Documentation/networking/filter.rst
+++ b/Documentation/networking/filter.rst
@@ -1053,6 +1053,33 @@ 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_SUB
+    BPF_AND
+    BPF_OR
+    BPF_XOR
+
+Each having isomorphic 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
+pre-modification value from memory.
+
+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 pre-modification
+value 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