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

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Message-Id: <4922E4D4.76E4.0078.0@novell.com>
Date:	Tue, 18 Nov 2008 14:52:52 +0000
From:	"Jan Beulich" <jbeulich@...ell.com>
To:	"Ingo Molnar" <mingo@...e.hu>
Cc:	<heukelum@...tmail.fm>, "Andi Kleen" <andi@...stfloor.org>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"Alexander van Heukelum" <heukelum@...lshack.com>,
	"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

>>> Ingo Molnar <mingo@...e.hu> 18.11.08 15:03 >>>
>* Jan Beulich <jbeulich@...ell.com> wrote:
>> 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 

This part I agree to.

>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.

Making gas auto-generate this is not really possible (much like ia64
requires the annotations to be inserted manually), mainly because gas
can't know whether e.g. a push of a register is in order to preserve its
value, or for some other purpose.

I do have a set of macros for this in nlkd, maybe (as you're asking for it)
I should get them out of there (and convert them to AT&T syntax).

Jan

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