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

To: David Daney <ddaney.cavm@gmail.com>
Subject: Re: [PATCH] mips: function tracer: Fix broken function tracing
From: Steven Rostedt <rostedt@goodmis.org>
Date: Tue, 15 Jan 2013 16:07:29 -0500
Cc: Al Cooper <alcooperx@gmail.com>, ralf@linux-mips.org, linux-mips@linux-mips.org, linux-kernel@vger.kernel.org
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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?

> 
> 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.

-- Steve



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