Re: [PATCH bpf-next v5 11/11] bpf: Document new atomic instructions

[Yonghong Song]

On 12/16/20 3:44 AM, Brendan Jackman wrote:
> On Wed, 16 Dec 2020 at 08:08, Yonghong Song <yhs@xxxxxx> wrote:
> > On 12/15/20 4:18 AM, Brendan Jackman wrote:
> > > Document new atomic instructions.
> > >
> > > Signed-off-by: Brendan Jackman <jackmanb@xxxxxxxxxx>
> >
> > Ack with minor comments below.
> >
> > Acked-by: Yonghong Song <yhs@xxxxxx>
> >
> > > ---
> > >    Documentation/networking/filter.rst | 26 ++++++++++++++++++++++++++
> > >    1 file changed, 26 insertions(+)
> > >
> > > diff --git a/Documentation/networking/filter.rst b/Documentation/networking/filter.rst
> > > index 1583d59d806d..26d508a5e038 100644
> > > --- a/Documentation/networking/filter.rst
> > > +++ b/Documentation/networking/filter.rst
> > > @@ -1053,6 +1053,32 @@ 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 (from architecture v4 onwards) are:
> >
> > Remove "(from architecture v4 onwards)".
>
> Oops, thanks.
>
> > > +
> > > +    BPF_ADD
> > > +    BPF_AND
> > > +    BPF_OR
> > > +    BPF_XOR
> > > +
> > > +Each having equivalent semantics with the ``BPF_ADD`` example, that is: the
> > > +memory location addresed by ``dst_reg + off`` is atomically modified, with
> > > +``src_reg`` as the other operand. If the ``BPF_FETCH`` flag is set in the
> > > +immediate, then these operations also overwrite ``src_reg`` with the
> > > +value that was in memory before it was modified.
> > > +
> > > +The more special operations are:
> > > +
> > > +    BPF_XCHG
> > > +
> > > +This atomically exchanges ``src_reg`` with the value addressed by ``dst_reg +
> > > +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``.
> > > +
> > >    Note that 1 and 2 byte atomic operations are not supported.
> >
> > Adding something like below.
> >
> > Except xadd for legacy reason, all other 4 byte atomic operations
> > require alu32 mode.
> > The alu32 mode can be enabled with clang flags "-Xclang -target-feature
> > -Xclang +alu32" or "-mcpu=v3". The cpu version 3 has alu32 mode on by
> > default.
>
> Thanks, I've written it as:
>
> 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``.

Sounds good. thanks!

> > >    You may encounter BPF_XADD - this is a legacy name for BPF_ATOMIC, referring to