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bpf: Document new atomic instructions

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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
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Brendan Jackman 2021-01-14 18:17:51 +00:00 committed by Alexei Starovoitov
parent 98d666d05a
commit de948576f8
1 changed files with 31 additions and 0 deletions
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Documentation/networking/filter.rst
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@ -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.