linux-kernel - Re: [PATCH][GIT PULL] x86, function-graph: only save return values on x86

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Date:	Wed, 25 Mar 2009 17:26:22 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Jaswinder Singh Rajput <jaswinder@...nel.org>
cc:	linux kernel <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>, Tim Bird <tim.bird@...sony.com>,
	Ingo Molnar <mingo@...hat.com>,
	Abhishek Sagar <sagar.abhishek@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH][GIT PULL] x86, function-graph: only save return values
 on x86_64


On Thu, 26 Mar 2009, Jaswinder Singh Rajput wrote:
> > 
> > diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
> > index a331ec3..1ac9986 100644
> > --- a/arch/x86/kernel/entry_64.S
> > +++ b/arch/x86/kernel/entry_64.S
> > @@ -147,27 +147,14 @@ END(ftrace_graph_caller)
> >  GLOBAL(return_to_handler)
> >  	subq  $80, %rsp
> >  
> > +	/* Save the return values */
> >  	movq %rax, (%rsp)
> > -	movq %rcx, 8(%rsp)
> > -	movq %rdx, 16(%rsp)
> > -	movq %rsi, 24(%rsp)
> > -	movq %rdi, 32(%rsp)
> > -	movq %r8, 40(%rsp)
> > -	movq %r9, 48(%rsp)
> > -	movq %r10, 56(%rsp)
> > -	movq %r11, 64(%rsp)
> > +	movq %rdx, 8(%rsp)
> >  
> >  	call ftrace_return_to_handler
> >  
> >  	movq %rax, 72(%rsp)
> > -	movq 64(%rsp), %r11
> > -	movq 56(%rsp), %r10
> > -	movq 48(%rsp), %r9
> > -	movq 40(%rsp), %r8
> > -	movq 32(%rsp), %rdi
> > -	movq 24(%rsp), %rsi
> > -	movq 16(%rsp), %rdx
> > -	movq 8(%rsp), %rcx
> > +	movq 8(%rsp), %rdx
> >  	movq (%rsp), %rax
> >  	addq $72, %rsp
> >  	retq
> 
> hmm, is this gonna work:
> 	movq %rax, 16(%rsp)
> 	movq 8(%rsp), %rdx
>  	movq (%rsp), %rax
>  	addq $16, %rsp
>  	retq

Interesting. I was just talking over IRC with Frederic about this. One 
theory about his earlier failures was not having a proper stack frame when 
calling the function. This is probably a bogus theory and there was 
probably something else that broke the code. But there is no harm in 
allocating 80 bytes (it worked before). Thus, I rather error on the side 
of least change than risk it.

-- Steve

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