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

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Message-ID: <20091123211945.GC4979@nowhere>
Date:	Mon, 23 Nov 2009 22:19:47 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
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

On Mon, Nov 23, 2009 at 07:09:39PM -0200, Arnaldo Carvalho de Melo wrote:
> 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.


Ok.

 
> 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


Hmm, but I worry a bit about alignment which would return us
the wrong size.

May be can we first try to get the address from /proc/kallsyms,
and if we have dwarf, get the size from it, otherwise try some
magic with /proc/kallsysms...

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