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

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Date:	Mon, 18 Jan 2010 19:04:45 +0530
From:	"K.Prasad" <prasad@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Avi Kivity <avi@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Mark Wielaard <mjw@...hat.com>,
	utrace-devel <utrace-devel@...hat.com>
Subject: Re: [RFC] [PATCH 1/7] User Space Breakpoint Assistance Layer (UBP)

On Mon, Jan 18, 2010 at 02:15:51PM +0100, Peter Zijlstra wrote:
> 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..
>

A very scarce resource that it is, well, sometimes all that we might have
is just one hw-breakpoint register (like older PPC64 with 1 IABR) in the
system. If one process/thread consumes it, then all other contenders (from
both kernel and user-space) are prevented from acquiring it.

Also to mention the existence of processors with no support for
instruction breakpoints.

Thanks,
K.Prasad

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