linux-kernel - Re: [RFC,v2] x86_64: save

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Date:	Tue, 18 Nov 2008 15:03:49 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Jan Beulich <jbeulich@...ell.com>
Cc:	Alexander van Heukelum <heukelum@...lshack.com>,
	heukelum@...tmail.fm, Andi Kleen <andi@...stfloor.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Glauber Costa <gcosta@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [RFC,v2] x86_64: save_args out of line

* Jan Beulich <jbeulich@...ell.com> wrote:

> >>> Alexander van Heukelum <heukelum@...lshack.com> 18.11.08 12:16 >>>
> >> >Dwarf2-annotations are most probably wrong or missing at all.
> >> 
> >> Indeed - do you have intentions to address this?
> >
> > Yes, I'ld like to get it right. What do you use to check the 
> > annotations?
> 
> No tool, if you mean that. Extensive changes I verify by looking at 
> the dump, problems are usually found only when back traces don't 
> come out right.

that's a fundamental weakness of all the CFI annotations.

It is outright wrong to waste humans on this mechanic task: as it is 
abundantly clear to GAS where we change a stack pointer and by how 
much - it could emit magic annotations automatically just as much.

So if you care about it, please fix this in the tools space. The 
entry_64.S impact of finegrained annotations is just too ugly for 
things like this.

One limited exception is for basic stack frames where we do syscalls 
or call into other C code. (i.e. the patch proposed here would have to 
do that limited annotation)

But the per instruction annotations currently in that code are madness 
and must either be cleaned up significantly via the use of GAS macros 
(so that all stack pointer manipulations go via a single macro 
invocation), or be completely auto-generated by GAS.

	Ingo
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