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

From: "Jose E. Marchesi" <jose.marchesi@oracle.com>
To: Yonghong Song <yonghong.song@linux.dev>
Cc: dthaler1968@googlemail.com,
	'Dave Thaler' <dthaler1968=40googlemail.com@dmarc.ietf.org>,
	bpf@vger.kernel.org, bpf@ietf.org
Subject: Re: [Bpf] [PATCH bpf-next v2] bpf, docs: Add callx instructions in new conformance group
Date: Tue, 13 Feb 2024 07:11:05 +0100	[thread overview]
Message-ID: <87zfw4sxl2.fsf@oracle.com> (raw)
In-Reply-To: <a81da29b-b671-484a-8f3d-743f1dac44f1@linux.dev> (Yonghong Song's message of "Mon, 12 Feb 2024 14:48:53 -0800")

> 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. The following is the llvm diff:
>
> https://github.com/llvm/llvm-project/pull/81546

Thank you.

I believe Will will be sending a patch to binutils to change the
pseudo-C syntax for the instruction to callx instead of callr.  We will
then adapt GCC accordingly, and both compilers will be doing exactly the
same regarding callx.

>>
>> 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.
>
>>
>> Dave
>>

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