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

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Date:   Tue, 15 Dec 2020 23:08:12 -0800
From:   Yonghong Song <yhs@...com>
To:     Brendan Jackman <jackmanb@...gle.com>, <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>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next v5 11/11] bpf: Document new atomic instructions



On 12/15/20 4:18 AM, Brendan Jackman wrote:
> Document new atomic instructions.
> 
> Signed-off-by: Brendan Jackman <jackmanb@...gle.com>

Ack with minor comments below.

Acked-by: Yonghong Song <yhs@...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)".

> +
> +    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.

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