linux-kernel - Re: [PATCH 4/4] perf tools: Add support for breakpoint events in perf tools

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Message-ID: <20091123210939.GD15547@ghostprotocols.net>
Date:	Mon, 23 Nov 2009 19:09:39 -0200
From:	Arnaldo Carvalho de Melo <acme@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	"K.Prasad" <prasad@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Paul Mackerras <paulus@...ba.org>
Subject: Re: [PATCH 4/4] perf tools: Add support for breakpoint events in
	perf tools

Em Mon, Nov 23, 2009 at 09:25:28PM +0100, Frederic Weisbecker escreveu:
> On Mon, Nov 23, 2009 at 11:06:01PM +0530, K.Prasad wrote:
> > With kernel symbols as input it would be possible to derive the bkpt
> > length based on the symbol-size, say using kallsyms_lookup_size_offset()
> > (although the corresponding length may not be available on the host
> > processor such requests can be failed or over-ridden by the user using
> > a smaller length), but for addresses I think it is vital to know what
> > breakpoint length is desired by the user.
 
> Yeah. I guess we first need a way to manually add this length, may be:

> 	mem:addr/len:access
 
> And as you said, finding it automatically for symbols. But still,
> passing symbols to perf attr leads to confusion and complexity if we
> want to profile in userspace.
 
> I think we should find this symbol length from userspace. I'm not sure
> how yet, probably using Dwarf. Arnaldo, do you have an idea about that?

DWARF has the type for each variable or struct member, getting its size
is straightforward.

Using just /proc/kallsyms all we can do is find the size of a variable
by looking at its address and the address of the next one.

- Arnaldo
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