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Date:	Mon, 5 Jan 2015 17:44:47 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Stephane Eranian <eranian@...gle.com>,
	Ingo Molnar <mingo@...hat.com>, Jiri Olsa <jolsa@...hat.com>,
	root <chenggang.qin@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	秦承刚(承刚) <chenggang.qcg@...bao.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Mike Galbraith <efault@....de>,
	Namhyung Kim <namhyung@...il.com>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	David Ahern <dsahern@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	秦承刚(承刚) 
	<chenggang.qcg@...baba-inc.com>,
	Yanmin Zhang <yanmin.zhang@...el.com>
Subject: Re: [PATCH 1/2] perf: Move task_pt_regs sampling into arch code

On Mon, Jan 05, 2015 at 08:13:49AM -0800, Andy Lutomirski wrote:
> > Just wondering how bad it would be to fill out the actual pt_regs that
> > was previously partially initialized?
> 
> Bad, for at least two reasons.
> 
> One is that we don't actually know which regs are initialized.  bx,
> bp, r12 etc are particularly bad in this regard, due to the FORK_LIKE
> mechanism and similar optimizations.

Right, but you need to deal with that anyhow.

> The other is that the uninitialized part of task_pt_regs can be used
> for something else entirely.  If we have a syscall instruction
> immediately followed by a regular interrupt, then the interrupt's
> hardware frame will overlap task_pt_regs.  (I'm not going to claim
> that this design is sensible, but it is what it is.  IIRC Denys
> Vlasenko had some patches to partially clean this up.)

Ah, urgh. Yes painful that.

> It would be possible to rework the code to avoid an extra pt_regs
> copy, but I don't see an obvious way to do it cleanly.

Yeah, we'll see how this works, I was just curious on the exact need for
the copy, but if as you say, the original structure might not even exist
properly (even though we have a pointer to it) that's bad (TM).
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