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

Steven Rostedt
2013-01-15T21:07:37Z

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