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

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Message-ID: <c8496a00-65d9-5231-d806-5b402c7fb3cf@fb.com>
Date:   Wed, 16 Dec 2020 07:25:59 -0800
From:   Yonghong Song <yhs@...com>
To:     Brendan Jackman <jackmanb@...gle.com>
CC:     bpf <bpf@...r.kernel.org>, 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>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next v5 11/11] bpf: Document new atomic instructions



On 12/16/20 3:44 AM, Brendan Jackman wrote:
> On Wed, 16 Dec 2020 at 08:08, Yonghong Song <yhs@...com> wrote:
>>
>>
>>
>> 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)".
> 
> 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
>>>