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Message-ID: <55652B7B.2040409@huawei.com>
Date:	Wed, 27 May 2015 10:27:07 +0800
From:	He Kuang <hekuang@...wei.com>
To:	Alexei Starovoitov <ast@...mgrid.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	<wangnan0@...wei.com>, <paulus@...ba.org>,
	<a.p.zijlstra@...llo.nl>, <mingo@...hat.com>, <acme@...nel.org>,
	<namhyung@...nel.org>, <jolsa@...nel.org>, <dsahern@...il.com>,
	<brendan.d.gregg@...il.com>, <daniel@...earbox.net>
CC:	<lizefan@...wei.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v2 09/15] perf probe: Support $params without
 debuginfo

hi, Alexei

On 2015/5/27 1:50, Alexei Starovoitov wrote:
> On 5/25/15 1:33 AM, He Kuang wrote:
>> Right, I learnt regparm(3) is mandatory in x86_32, according to rules,
>> the first three args will go to regparm(ax, dx, cx). But we should not
>> refer arg1~3 to ax, dx, cx because of 64bit parameters (other reasons?).
>>
>> Consider this keyword is used for generating bpf prologue which fetches
>> formal parameters when no debuginfo is provided, for this purpose, we can:
>> 	1) We just help fetch the $regs or $regparms(If the keyword is
>> $regparms, ax/dx/cx is fetched, nothing related to args) to bpf arglists
>> and leave the rest things to bpf prog writer.
>>
>> 	2) Keep that on platforms like x86_64 and skip this feature on
>> platforms like x86_32.
>>
>> or any other suggestions?
> 
> Single argument like $regparam or whatever name cannot work on all
> architectures, that's why in the very beginning I suggested
> 'func(long, char, void*)' syntax to describe arguments when debuginfo
> is not available. Calling convention for scalars is simple enough on
> all major architectures. x64_64 - trivial, i64_32 - a bit more involved,
> but simple enough so that list of types of arguments is enough to figure
> out which register or register pair or stack should be used to fetch
> argN.
> 
> 
As Masami has reminded,  the use of 'asmlinkage' forces regparm=0, and
we can't destinguish them without debuginfo, so 'func(long, char,
void*)' syntax not work in everywhere.

In fact, all the context infos are there in bpf prog(pt_regs in arg1).
To the non-debuginfo case, without the help of prologue, user steps
following flow to fetch params:

1. pt_regs(arg1) + architecture => calling regs

2. calling regs + function prototype(SEC) + gcc attributes(like
asmlinkage) => formal parameters

 '$regparms' do the 1st step, though not a full workaround. But for the
lack of gcc attributes, it seems we can't do the 2nd step. Any ideas?

Thanks

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