linux-kernel - Re: [RFC] [PATCH 1/7] User Space Breakpoint Assistance Layer (UBP)

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Date:	Mon, 18 Jan 2010 14:15:51 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Avi Kivity <avi@...hat.com>
Cc:	ananth@...ibm.com, Jim Keniston <jkenisto@...ibm.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	utrace-devel <utrace-devel@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Maneesh Soni <maneesh@...ibm.com>,
	Mark Wielaard <mjw@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] [PATCH 1/7] User Space Breakpoint Assistance Layer (UBP)

On Mon, 2010-01-18 at 14:37 +0200, Avi Kivity wrote:
> On 01/18/2010 02:14 PM, Peter Zijlstra wrote:
> >
> >> Well, the alternatives are very unappealing.  Emulation and
> >> single-stepping are going to be very slow compared to a couple of jumps.
> >>      
> > With CPL2 or RPL on user segments the protection issue seems to be
> > manageable for running the instructions from kernel space.
> >    
> 
> CPL2 gives unrestricted access to the kernel address space; and RPL does 
> not affect page level protection.  Segment limits don't work on x86-64.  
> But perhaps I missed something - these things are tricky.

So setting RPL to 3 on the user segments allows access to kernel pages
just fine? How useful.. :/

> It should be possible to translate the instruction into an address space 
> check, followed by the action, but that's still slower due to privilege 
> level switches.

Well, if you manage to do the address validation you don't need the priv
level switch anymore, right?

Are the ins encodings sane enough to recognize mem parameters without
needing to know the actual ins?

How about using a hw-breakpoint to close the gap for the inline single
step? You could even re-insert the int3 lazily when you need the
hw-breakpoint again. It would consume one hw-breakpoint register for
each task/cpu that has probes though..

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