Re: Encoding of V4 32-bit JA

[Yonghong Song <yhs@meta.com>]

On 7/23/23 12:21 PM, Eduard Zingerman wrote:
> On Sun, 2023-07-23 at 21:14 +0200, Jose E. Marchesi wrote:
>>> On Fri, 2023-07-21 at 18:19 +0200, Jose E. Marchesi wrote:
>>>> Hi Yonghong.
>>>>
>>>> This is from the v4 instructions proposal:
>>>>
>>>>      ========  =====  =========================  ============
>>>>      code      value  description                notes
>>>>      ========  =====  =========================  ============
>>>>      BPF_JA    0x00   PC += imm                  BPF_JMP32 only
>>>>
>>>> Is this instruction using source 1 instead of 0?  Otherwise, it would
>>>> have exactly the same encoding than the V3< JA instruction.  Is that
>>>> what is intended?
>>>>
>>>> TIA.
>>>>
>>>
>>> Hi Jose,
>>>
>>> I think that assumption is that `BPF_JMP32 | BPF_JA` is currently free:
>>> - documentation [1] implies that only `BPF_JMP` should be used for `BPF_JA`
>>>    (see "notes" column for the first line)
>>> - BPF verifier rejects `BPF_JMP32 | BPF_JA`
>>> - clang always generates `BPF_JMP | BPF_JA`
>>
>> Makes sense, thanks for the info.
>>
>> Do you know the precise pseudo-c assembly syntax to use for this
>> instruction?
> 
> In [1] Yonghong uses the following form:
> 
>    gotol +0xcd9b
> 
> But it seems to be not specified in the documentation for the patch-set v3.

Thanks Eduard, I will add gotol to the documentation.

> 
> [1] https://reviews.llvm.org/D144829
> 
>>
>>> Thanks,
>>> Eduard
>>>
>>> [1] https://www.kernel.org/doc/html/latest/bpf/instruction-set.html#jump-instructions
>