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| author |   Brendan Jackman <jackmanb@google.com> | 2021-01-14 18:17:51 +0000 |
|---|---|---|
| committer |   Alexei Starovoitov <ast@kernel.org> | 2021-01-14 18:34:30 -0800 |
| commit | de948576f8e7d7fa1b5db04f56184ffe176177c5 (patch) | |
| tree | cb057d167ff8292f09cd4a4e78f4bd610a6fa959 /Documentation | |
| parent | bpf: Add tests for new BPF atomic operations (diff) | |
| download | linux-dev-de948576f8e7d7fa1b5db04f56184ffe176177c5.tar.xz linux-dev-de948576f8e7d7fa1b5db04f56184ffe176177c5.zip | |
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
Diffstat (limited to 'Documentation')
| -rw-r--r-- | Documentation/networking/filter.rst | 31 |
1 files changed, 31 insertions, 0 deletions
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: + + 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. |