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

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

[Brendan Jackman]

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@google.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

Re: [PATCH bpf-next] docs: bpf: Fixup atomics documentation

[Jonathan Corbet]

On Mon, 18 Jan 2021 11:36:43 +0000
Brendan Jackman <jackmanb@google.com> wrote:

> 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.

This starts to sound like material for more than one patch...?

> 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.

And why did you do that?  

> [1] https://lore.kernel.org/bpf/CA+i-1C3cEXqxcXfD4sibQfx+dtmmzvOzruhk8J5pAw3g5v=KgA@mail.gmail.com/T/#t
> 
> Signed-off-by: Brendan Jackman <jackmanb@google.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: ::

Things like this read really strangely.  Just say "encoding::" and be done
with it, please.

Thanks,

jon

Re: [PATCH bpf-next] docs: bpf: Fixup atomics documentation

[Brendan Jackman]

Thanks for the review :)

On Mon, 18 Jan 2021 at 16:33, Jonathan Corbet <corbet@lwn.net> wrote:
>
> On Mon, 18 Jan 2021 11:36:43 +0000
> Brendan Jackman <jackmanb@google.com> wrote:
>
> > 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.
>
> This starts to sound like material for more than one patch...?

Good point, I'll split the markup fixups and actual content change
into separate patches.

> > 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.
>
> And why did you do that?

Hmm, indeed looks like that isn't necessary as long as there are no
spaces between the previous character and the '::'.

v2 incoming...

> > [1] https://lore.kernel.org/bpf/CA+i-1C3cEXqxcXfD4sibQfx+dtmmzvOzruhk8J5pAw3g5v=KgA@mail.gmail.com/T/#t
> >
> > Signed-off-by: Brendan Jackman <jackmanb@google.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: ::
>
> Things like this read really strangely.  Just say "encoding::" and be done
> with it, please.
>
> Thanks,
>
> jon