linux-kernel - Re: [RFC v3 0/8] x86, xsave: rework of extended state handling, LWP support

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Date:	Wed, 18 May 2011 13:22:59 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Hans Rosenfeld <hans.rosenfeld@....com>
Cc:	"hpa@...or.com" <hpa@...or.com>, "x86@...nel.org" <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Richter, Robert" <robert.richter@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [RFC v3 0/8] x86, xsave: rework of extended state handling, LWP
 support


* Hans Rosenfeld <hans.rosenfeld@....com> wrote:

> > Here are a couple of suggestions to LWP hardware designers:
> > 
> >  - the fact that LWP cannot count kernel events right now is unfortunate - 
> >    there's no reason not to allow privileged user-space to request ring 3
> >    events as well - hopefully this misfeature will be fixed in future 
> >    iterations of the hardware.
> > 
> >  - it would be nice to allow the per task masking/unmasking of LWP without
> >    having to modify the cr0 (which can be expensive). A third mode
> >    implemented in the LWP_CFG MSG would suffice: it would make the LWP
> >    instructions privileged, but would otherwise allow LWP event collection
> >    to occur even on sandboxed code.
> > 
> >  - it would be nice to also log the previous retired instruction in the
> >    trace entry, to ease decoding of the real instruction that generated
> >    an event. (Fused instructions can generate their RIP at the first
> >    instruction.)
> 
> I will forward this to our hardware designers, but I have my doubts about the 
> first two of your suggestions. They seem to be orthogonal to what LWP is 
> supposed to be.

Not sure why you think those two suggestions are 'orthogonal to LWP', they are 
not:

 - the second suggestion adds a third security model to the current 
   all-or-nothing nature of LWP instructions.

 - the first suggestion is a variation of its current security model as well:
   it allows LWP driven event collection in kernel mode, not just user mode.

There is nothing fundamentally ring-3-only about the concept of 'light weight 
profiling' - while ring-3-only event collection is understandably necessary for 
unprivileged user-space, it is not the only interesting mode of lightweight 
event collection.

Thanks,

	Ingo
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