Encoding of V4 32-bit JA

Encoding of V4 32-bit JA

[Jose E. Marchesi]

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.

Re: Encoding of V4 32-bit JA

[Eduard Zingerman]

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`

Thanks,
Eduard

[1] https://www.kernel.org/doc/html/latest/bpf/instruction-set.html#jump-instructions

Re: Encoding of V4 32-bit JA

[Jose E. Marchesi]

> 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?

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

Re: Encoding of V4 32-bit JA

[Eduard Zingerman]

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.

[1] https://reviews.llvm.org/D144829

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

Re: Encoding of V4 32-bit JA

[Jose E. Marchesi]

> 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.

I will use that syntax in binutils for now.

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

Re: Encoding of V4 32-bit JA

[Jose E. Marchesi]

>> 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.
>
> I will use that syntax in binutils for now.

https://sourceware.org/pipermail/binutils/2023-July/128543.html

Now we will make GCC to use that instruction whenever needed, but only
with -mcpu=v4 or later:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110781

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

Re: Encoding of V4 32-bit JA

[Yonghong Song]

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
>