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Date:	Wed, 21 Oct 2009 17:14:37 +0400
From:	Sergei Shtylyov <sshtylyov@ru.mvista.com>
Organization: MontaVista Software Inc.
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To:	rostedt@goodmis.org
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	Nicholas Mc Guire <der.herr@hofr.at>,
	Ralf Baechle <ralf@linux-mips.org>, linux-mips@linux-mips.org,
	linux-kernel@vger.kernel.org
Subject: Re: ftrace for MIPS
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Hello.

Steven Rostedt wrote:

>>Need to check which registers is missing when saving/restoring for
>>_mcount:

>>NESTED(ftrace_graph_caller, PT_SIZE, ra) 
>>        MCOUNT_SAVE_REGS
>>        PTR_S   v0, PT_R2(sp)
>>
>>        MCOUNT_SET_ARGS
>>        jal     prepare_ftrace_return
>>        nop
>>
>>        /* overwrite the parent as &return_to_handler: v0 -> $1(at) */
>>        move    $1,     v0  

> I'm confused here? I'm not exactly sure what the above is doing. Is $1 a
> register (AT)?

    Yes.

> And how is this register used before calling mcount?

>>        PTR_L   v0, PT_R2(sp)
>>        MCOUNT_RESTORE_REGS
>>        RETURN_BACK
>>        END(ftrace_graph_caller)

>>        .align  2
>>        .globl  return_to_handler
>>return_to_handler:
>>        PTR_SUBU        sp, PT_SIZE
>>        PTR_S   v0, PT_R2(sp)

> BTW, is v0 the only return register? I know x86 can return two different
> registers depending on what it returns. What happens if a function
> returns a 64 bit value on a 32bit box? Does it use two registers for
> that?

    Yes, there's also v1 register.

WBR, Sergei