linux-kernel - [PATCH bpf-next] docs: bpf: Fixup atomics documentation

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Message-Id: <20210118113643.232579-1-jackmanb@google.com>
Date:   Mon, 18 Jan 2021 11:36:43 +0000
From:   Brendan Jackman <jackmanb@...gle.com>
To:     bpf@...r.kernel.org
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Florent Revest <revest@...omium.org>,
        Lukas Bulwahn <lukas.bulwahn@...il.com>,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        Jonathan Corbet <corbet@....net>,
        Brendan Jackman <jackmanb@...gle.com>
Subject: [PATCH bpf-next] docs: bpf: Fixup atomics documentation

This fixues up the markup to fix a warning, be more consistent with
use of monospace, and use the correct .rst syntax for <em> (* instead
of _). It also clarifies the explanation of Clang's -mcpu
requirements for this feature, Alexei pointed out that use of the
word "version" was confusing here.

NB this conflicts with Lukas' patch at [1], here where I've added
`::` to fix the warning, I also kept the original ':' which appears
in the output text.

[1] https://lore.kernel.org/bpf/CA+i-1C3cEXqxcXfD4sibQfx+dtmmzvOzruhk8J5pAw3g5v=KgA@mail.gmail.com/T/#t

Signed-off-by: Brendan Jackman <jackmanb@...gle.com>
---
 Documentation/networking/filter.rst | 30 +++++++++++++++--------------
 1 file changed, 16 insertions(+), 14 deletions(-)

diff --git a/Documentation/networking/filter.rst b/Documentation/networking/filter.rst
index f6d8f90e9a56..ba03e90a9163 100644
--- a/Documentation/networking/filter.rst
+++ b/Documentation/networking/filter.rst
@@ -1048,12 +1048,12 @@ Unlike classic BPF instruction set, eBPF has generic load/store operations::
 Where size is one of: BPF_B or BPF_H or BPF_W or BPF_DW.
 
 It also includes atomic operations, which use the immediate field for extra
-encoding.
+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:
+The basic atomic operations supported are: ::
 
     BPF_ADD
     BPF_AND
@@ -1066,12 +1066,12 @@ memory location addresed by ``dst_reg + off`` is atomically modified, with
 immediate, then these operations also overwrite ``src_reg`` with the
 value that was in memory before it was modified.
 
-The more special operations are:
+The more special operations are: ::
 
     BPF_XCHG
 
 This atomically exchanges ``src_reg`` with the value addressed by ``dst_reg +
-off``.
+off``. ::
 
     BPF_CMPXCHG
 
@@ -1081,19 +1081,21 @@ 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``.
+Clang can generate atomic instructions when ``-mcpu=v3`` is enabled (this is the
+default). 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, referring to
-the exclusive-add operation encoded when the immediate field is zero.
+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.
 
-eBPF has one 16-byte instruction: BPF_LD | BPF_DW | BPF_IMM which consists
+eBPF has one 16-byte instruction: ``BPF_LD | BPF_DW | BPF_IMM`` which consists
 of two consecutive ``struct bpf_insn`` 8-byte blocks and interpreted as single
-instruction that loads 64-bit immediate value into a dst_reg.
-Classic BPF has similar instruction: BPF_LD | BPF_W | BPF_IMM which loads
-32-bit immediate value into a register.
+instruction that loads 64-bit immediate value into a dst_reg.  Classic BPF has
+similar instruction: ``BPF_LD | BPF_W | BPF_IMM`` which loads 32-bit immediate
+value into a register.
 
 eBPF verifier
 -------------

base-commit: 232164e041e925a920bfd28e63d5233cfad90b73
-- 
2.30.0.284.gd98b1dd5eaa7-goog