linux-kernel - Re: [PATCH] mips: function tracer: Fix broken function tracing

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Date:	Tue, 15 Jan 2013 13:34:48 -0800
From:	David Daney <ddaney.cavm@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Al Cooper <alcooperx@...il.com>, ralf@...ux-mips.org,
	linux-mips@...ux-mips.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mips: function tracer: Fix broken function tracing

On 01/15/2013 01:07 PM, Steven Rostedt wrote:
> On Tue, 2013-01-15 at 09:55 -0800, David Daney wrote:
>
>>> There's nothing that states what the ftrace caller must be. We can have
>>> it do a proper stack update. That is, only at boot up do we need to
>>> handle the defined mcount. After that, those instructions are just place
>>> holders for our own algorithms. If the addiu was needed for the defined
>>> mcount, there's no reason to keep it for our own ftrace_caller.
>>>
>>> Would that work?
>>
>> ... either do as you suggest and dynamically change the ABI of the
>> target function.
>
> We already change the ABI. We have it call ftrace_caller instead of
> mcount.
>
> BTW, I've just compiled with gcc 4.6.3 against mips, and I don't see the
> issue. I have:
>
> 0000000000000000 <account_kernel_stack>:
>         0:       03e0082d        move    at,ra
>         4:       0c000000        jal     0 <account_kernel_stack>
>                          4: R_MIPS_26    _mcount
>                          4: R_MIPS_NONE  *ABS*
>                          4: R_MIPS_NONE  *ABS*
>         8:       0000602d        move    t0,zero
>         c:       2402000d        li      v0,13
>        10:       3c030000        lui     v1,0x0
>                          10: R_MIPS_HI16 mem_section
>                          10: R_MIPS_NONE *ABS*
>                          10: R_MIPS_NONE *ABS*
>        14:       000216fc        dsll32  v0,v0,0x1b
>        18:       64630000        daddiu  v1,v1,0
>
> Is it dependent on the config?

Yes.

You need to select a 32-bit kernel (which in turn may require selecting 
a board type that also supports it).

The ABI is different for 32-bit and 64-bit _mcount.

David Daney


>
>>
>> Or add support to GCC for a better tracing ABI (as I already said we did
>> for mips64).
>
> I wouldn't waste time changing gcc for this. If you're going to change
> gcc than please implement the -mfentry option. Look at x86_64 to
> understand this more.

A good point.  But I don't really plan on doing any work related to 
32-bit mips things at this point, so any such change would have to be 
done by someone else.

David Daney

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