Re: [PATCH -v4 4/9] tracing: add static function tracer support for MIPS

[David Daney <ddaney@caviumnetworks.com>]

Wu Zhangjin wrote:
> On Wed, 2009-10-21 at 11:24 -0400, Steven Rostedt wrote:
[...]
>>> +
>>> +NESTED(_mcount, PT_SIZE, ra)
>>> +	RESTORE_SP_FOR_32BIT
>>> +	PTR_LA	t0, ftrace_stub
>>> +	PTR_L	t1, ftrace_trace_function /* please don't use t1 later, safe? */
>> Is t0 and t1 safe for mcount to use? Remember, mcount does not follow
>> the dynamics of C function ABI.
> 
> So, perhaps we can use the saved registers(a0,a1...) instead.
> 

a0..a7 may not always be saved.

You can use at, v0, v1 and all the temporary registers.  Note that for 
the 64-bit ABIs sometimes the names t0-t4 shadow a4-a7.  So for a 64-bit 
kernel, you can use: $1, $2, $3, $12, $13, $14, $15, $24, $25, noting 
that at == $1 and contains the callers ra.  For a 32-bit kernel you can 
add $8, $9, $10, and $11

This whole thing seems a little fragile.

I think it might be a good idea to get input from Richard Sandiford, 
and/or Adam Nemet about this approach (so I add them to the CC).

This e-mail thread starts here:

http://www.linux-mips.org/archives/linux-mips/2009-10/msg00286.html

and here:

http://www.linux-mips.org/archives/linux-mips/2009-10/msg00290.html

David Daney


> Regards!
> 
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