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

[He Kuang]

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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