linux-kernel - Re: [PATCH 1/3] x86: record relocation offset

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Date:	Fri, 1 Jan 2010 10:27:33 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Arnaldo Carvalho de Melo <acme@...radead.org>
Cc:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	James Bottomley <James.Bottomley@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	"Frank Ch. Eigler" <fche@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] x86: record relocation offset


* Arnaldo Carvalho de Melo <acme@...radead.org> wrote:

> Em Thu, Dec 31, 2009 at 11:00:00AM +0800, Xiao Guangrong escreveu:
> > Arnaldo Carvalho de Melo wrote:
> 
> > > Conclusion: at 'perf record' time store the address of a well know
> > > symbol (_text) into the perf.data header. Later, at perf report time, if
> > > using a vmlinux file, calculate the relocation by subtracting the same
> > > well known symbol from the one stored in the header.
> 
> > > So no need for ioctl or boot stuff.
> 
> > I'm little confused, how to get the load symbol address?
> > It's not a good way, if you get it from '/proc/kallsyms', we can't assume kernel
> > has this file.
> 
> Well, then its just a matter of exposing _text as 
> /sys/kernel/sections/.text, as we already have for modules:
> 
> [acme@ana linux-2.6-tip]$ cat /sys/module/ipv6/sections/.text 
> 0xfa0c2000
> 
> Which matches
> 
> nf_conntrack_ipv6 17548 2 - Live 0xfa147000
> ipv6 239420 32 ip6t_REJECT,nf_conntrack_ipv6, Live 0xfa0c2000
> [acme@ana linux-2.6-tip]$

Yeah, that's a good idea and pretty complementary to the existing scheme for 
modules.

> But even as a quick transational assist, we can look at kallsyms at 'perf 
> record' time.

Yes, we should do that as a fallback mechanism.

(Initially this 'fallback' will be the primary method, until the 
sections/.text extension gets merged upstream.)

Thanks,

	Ingo
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