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

From: Brendan Jackman <jackmanb@google.com>
To: Yonghong Song <yhs@fb.com>
Cc: bpf <bpf@vger.kernel.org>, 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>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH bpf-next v5 11/11] bpf: Document new atomic instructions
Date: Wed, 16 Dec 2020 12:44:45 +0100	[thread overview]
Message-ID: <CA+i-1C2ddNES0DXoO1L_nrqpK5EtA9xKE1yRGrqSVv0dECZozQ@mail.gmail.com> (raw)
In-Reply-To: <fcb9335c-000c-0097-7a70-983de271a3b7@fb.com>

On Wed, 16 Dec 2020 at 08:08, Yonghong Song <yhs@fb.com> wrote:
>
>
>
> On 12/15/20 4:18 AM, Brendan Jackman wrote:
> > Document new atomic instructions.
> >
> > Signed-off-by: Brendan Jackman <jackmanb@google.com>
>
> Ack with minor comments below.
>
> Acked-by: Yonghong Song <yhs@fb.com>
>
> > ---
> >   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``.

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