From: Brendan Jackman <jackmanb@google.com>
To: bpf@vger.kernel.org
Cc: Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
Andrii Nakryiko <andrii.nakryiko@gmail.com>,
KP Singh <kpsingh@chromium.org>,
Florent Revest <revest@chromium.org>,
Lukas Bulwahn <lukas.bulwahn@gmail.com>,
linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
Jonathan Corbet <corbet@lwn.net>,
Brendan Jackman <jackmanb@google.com>
Subject: [PATCH bpf-next v2 2/2] docs: bpf: Clarify -mcpu=v3 requirement for atomic ops
Date: Mon, 18 Jan 2021 15:57:35 +0000 [thread overview]
Message-ID: <20210118155735.532663-3-jackmanb@google.com> (raw)
In-Reply-To: <20210118155735.532663-1-jackmanb@google.com>
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/T/#m07264fc18fdc43af02fc1320968afefcc73d96f4
Signed-off-by: Brendan Jackman <jackmanb@google.com>
---
Documentation/networking/filter.rst | 7 ++++---
1 file 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``,
--
2.30.0.284.gd98b1dd5eaa7-goog
next prev parent reply other threads:[~2021-01-18 16:01 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-18 15:57 [PATCH bpf-next v2 0/2] BPF docs fixups Brendan Jackman
2021-01-18 15:57 ` [PATCH bpf-next v2 1/2] docs: bpf: Fixup atomics markup Brendan Jackman
2021-01-18 17:17 ` Lukas Bulwahn
2021-01-18 15:57 ` Brendan Jackman [this message]
2021-01-18 17:36 ` [PATCH bpf-next v2 2/2] docs: bpf: Clarify -mcpu=v3 requirement for atomic ops Yonghong Song
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