Re: [Bpf] [PATCH bpf-next v2] bpf, docs: Add callx instructions in new conformance group

From: Yonghong Song <yonghong.song@linux.dev>
To: dthaler1968@googlemail.com,
	"'Jose E. Marchesi'" <jose.marchesi@oracle.com>
Cc: bpf@vger.kernel.org, bpf@ietf.org
Subject: Re: [Bpf] [PATCH bpf-next v2] bpf, docs: Add callx instructions in new conformance group
Date: Mon, 12 Feb 2024 17:23:47 -0800	[thread overview]
Message-ID: <d3b89acf-fb83-4bdf-84e1-6c4a77f3ad36@linux.dev> (raw)
In-Reply-To: <03a801da5e1a$8d0274c0$a7075e40$@gmail.com>


On 2/12/24 5:18 PM, dthaler1968@googlemail.com wrote:
>> -----Original Message-----
>> From: Yonghong Song <yonghong.song@linux.dev>
>> Sent: Monday, February 12, 2024 2:49 PM
>> To: dthaler1968@googlemail.com; 'Jose E. Marchesi'
>> <jose.marchesi@oracle.com>; 'Dave Thaler'
>> <dthaler1968=40googlemail.com@dmarc.ietf.org>
>> Cc: bpf@vger.kernel.org; bpf@ietf.org
>> Subject: Re: [Bpf] [PATCH bpf-next v2] bpf, docs: Add callx instructions in new
>> conformance group
>>
>>
>> On 2/12/24 1:52 PM, dthaler1968@googlemail.com wrote:
>>>> -----Original Message-----
>>>> From: Yonghong Song <yonghong.song@linux.dev>
>>>> Sent: Monday, February 12, 2024 1:49 PM
>>>> To: Jose E. Marchesi <jose.marchesi@oracle.com>; Dave Thaler
>>>> <dthaler1968=40googlemail.com@dmarc.ietf.org>
>>>> Cc: bpf@vger.kernel.org; bpf@ietf.org; Dave Thaler
>>>> <dthaler1968@gmail.com>
>>>> Subject: Re: [Bpf] [PATCH bpf-next v2] bpf, docs: Add callx
>>>> instructions in new conformance group
>>>>
>>>>
>>>> On 2/12/24 1:28 PM, Jose E. Marchesi wrote:
>>>>>> +BPF_CALL  0x8    0x1  call PC += reg_val(imm)          BPF_JMP | BPF_X
>>>> only, see `Program-local functions`_
>>>>> If the instruction requires a register operand, why not using one of
>>>>> the register fields?  Is there any reason for not doing that?
>>>> Talked to Alexei and we think using dst_reg for the register for
>>>> callx insn is better. I will craft a llvm patch for this today. Thanks!
>>> Why dst_reg instead of src_reg?
>>> BPF_X is supposed to mean use src_reg.
>> Let us use dst_reg. Currently, for BPF_K, we have src_reg for a bunch of flags
>> (pseudo call, kfunc call, etc.). So for BPF_X, let us preserve this property as
>> well in case in the future we will introduce variants for callx.
> Ah yes, that makes sense.
>
>> The following is the llvm diff:
>>
>> https://github.com/llvm/llvm-project/pull/81546
> Which llvm release is it targeted for?
> 18.1.0-rc3? 18.1.1?  later?

llvm19

>
>>> But this thread is about reserving/documenting the existing practice,
>>> since anyone trying to use it would run into interop issues because
>>> of existing clang.   Should we document both and list one as deprecated?
>> I think just documenting the new encoding is good enough. But other
>> people can chime in just in case that I missed something.
> Ok.
>
> Dave
>