[PATCH bpf-next] docs: bpf: Clarify BPF_CMPXCHG wording

[PATCH bpf-next] docs: bpf: Clarify BPF_CMPXCHG wording

[Ilya Leoshkevich]

Based on [1], BPF_CMPXCHG should always load the old value into R0. The
phrasing in bpf.rst is somewhat ambiguous in this regard, improve it to
make this aspect crystal clear.

  [1] https://lore.kernel.org/bpf/CAADnVQJFcFwxEz=wnV=hkie-EDwa8s5JGbBQeFt1TGux1OihJw@mail.gmail.com/

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
 Documentation/networking/filter.rst | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/Documentation/networking/filter.rst b/Documentation/networking/filter.rst
index b3f457802836..251c6bd73d15 100644
--- a/Documentation/networking/filter.rst
+++ b/Documentation/networking/filter.rst
@@ -1076,8 +1076,8 @@ 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``.
+``R0``. If they match it is replaced with ``src_reg``. In either case, the
+value that was there before is zero-extended and loaded back to ``R0``.
 
 Note that 1 and 2 byte atomic operations are not supported.
 
-- 
2.29.2

Re: [PATCH bpf-next] docs: bpf: Clarify BPF_CMPXCHG wording

[Alexei Starovoitov]

On Wed, Feb 10, 2021 at 6:29 AM Ilya Leoshkevich <iii@linux.ibm.com> wrote:
>
> Based on [1], BPF_CMPXCHG should always load the old value into R0. The
> phrasing in bpf.rst is somewhat ambiguous in this regard, improve it to
> make this aspect crystal clear.
>
>   [1] https://lore.kernel.org/bpf/CAADnVQJFcFwxEz=wnV=hkie-EDwa8s5JGbBQeFt1TGux1OihJw@mail.gmail.com/
>
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>

Applied.