Re: [PATCH] mips: function tracer: Fix broken function tracing

David Daney
2013-01-15T21:34:56Z

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