linux-kernel - Re: [RFC PATCH v2 09/15] perf probe: Support $params without debuginfo

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Message-ID: <5565ADC6.9070407@hitachi.com>
Date:	Wed, 27 May 2015 20:43:02 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	He Kuang <hekuang@...wei.com>,
	Alexei Starovoitov <ast@...mgrid.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

On 2015/05/27 11:27, He Kuang wrote:
> 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?

If you don't have the debuginfo, $regparams will help, but not cover
all the cases. This just means users may need to take care of using it.
Actually, in most cases, I'm sure $regparams will work fine, since
most functions are not asmlinkage'ed.

If you consider bpf requires correct parameters, you need debuginfo or
something like it, e.g. the pre-analyzed cache of perf-probe (see
https://lkml.org/lkml/2014/10/31/207).

Thank you,

-- 
Masami HIRAMATSU
Linux Technology Research Center, System Productivity Research Dept.
Center for Technology Innovation - Systems Engineering
Hitachi, Ltd., Research & Development Group
E-mail: masami.hiramatsu.pt@...achi.com
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