linux-kernel - Re: [RFC PATCH 2/9] perf: Add ability to dump user regs

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Message-ID: <20101013145611.GC5335@nowhere>
Date:	Wed, 13 Oct 2010 16:56:14 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Stephane Eranian <eranian@...gle.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Tom Zanussi <tzanussi@...il.com>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Robert Richter <robert.richter@....com>,
	David Miller <davem@...emloft.net>
Subject: Re: [RFC PATCH 2/9] perf: Add ability to dump user regs

On Wed, Oct 13, 2010 at 09:20:53AM +0200, Peter Zijlstra wrote:
> On Wed, 2010-10-13 at 07:06 +0200, Frederic Weisbecker wrote:
> > Add new PERF_SAMPLE_UREGS to perf sample type. This will dump the
> > user space context as it was before the user entered the kernel for
> > whatever reason.
> > 
> > This is going to be useful to bring Dwarf CFI based stack unwinding
> > on top of samples.
> 
> This doesn't address any of the issues that were raised previously.
> 
> There's a reason we don't have PERF_SAMPLE_*REGS like things.
> 
> See: http://lkml.org/lkml/2010/3/3/308


Right, we indeed have no way currently to know where these regs came
from. So we need to dump the kernel arch informations.

Probably we should do that from the kernel, so that compat archs
really can't get it wrong.

But I suspect we should do it once and not on every sample, so it
must be no sample type.

A silly idea would be to implement a minimal counter which counting
value gives a code that tells about the arch. No sample at all for this
event, just a count.

Hmm?

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