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| author |   Brendan Jackman <jackmanb@google.com> | 2021-01-20 13:39:46 +0000 |
|---|---|---|
| committer |   Alexei Starovoitov <ast@kernel.org> | 2021-01-20 17:03:21 -0800 |
| commit | b452ee005a9135ed89fc8c9dff14e042770eb4f1 (patch) | |
| tree | 4cea1a22cf1ea9c9b6ffc275e5a0d74e6d2722ca | |
| parent | docs: bpf: Fixup atomics markup (diff) | |
| download | linux-dev-b452ee005a9135ed89fc8c9dff14e042770eb4f1.tar.xz linux-dev-b452ee005a9135ed89fc8c9dff14e042770eb4f1.zip | |
docs: bpf: Clarify -mcpu=v3 requirement for atomic ops
Alexei pointed out [1] that this wording is pretty confusing. Here's
an attempt to be more explicit and clear.
[1] https://lore.kernel.org/bpf/CAADnVQJVvwoZsE1K+6qRxzF7+6CvZNzygnoBW9tZNWJELk5c=Q@mail.gmail.com/
Signed-off-by: Brendan Jackman <jackmanb@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210120133946.2107897-3-jackmanb@google.com
| -rw-r--r-- | Documentation/networking/filter.rst | 7 |
1 files changed, 4 insertions, 3 deletions
diff --git a/Documentation/networking/filter.rst b/Documentation/networking/filter.rst index 4c2bb4c6364d..b3f457802836 100644 --- a/Documentation/networking/filter.rst +++ b/Documentation/networking/filter.rst @@ -1081,9 +1081,10 @@ 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 +Clang can generate atomic instructions by default when ``-mcpu=v3`` is +enabled. If a lower version for ``-mcpu`` is set, the only atomic instruction +Clang can generate is ``BPF_ADD`` *without* ``BPF_FETCH``. If you need to enable +the atomics features, while keeping a lower ``-mcpu`` version, you can use ``-Xclang -target-feature -Xclang +alu32``. You may encounter ``BPF_XADD`` - this is a legacy name for ``BPF_ATOMIC``, |