linux-kernel - Re: [RFC][PATCH 1/5] hw-breakpoints: Make kernel breakpoints API truly generic

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Message-ID: <20090728013500.GE5147@nowhere>
Date:	Tue, 28 Jul 2009 03:35:01 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...uxtronix.de>,
	Mike Galbraith <efault@....de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Anton Blanchard <anton@...ba.org>,
	Li Zefan <lizf@...fujitsu.com>,
	Zhaolei <zhaolei@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	"K . Prasad" <prasad@...ux.vnet.ibm.com>,
	Alan Stern <stern@...land.harvard.edu>
Subject: Re: [RFC][PATCH 1/5] hw-breakpoints: Make kernel breakpoints API
	truly generic

On Sat, Jul 25, 2009 at 11:38:49AM -0400, Mathieu Desnoyers wrote:
> * Frederic Weisbecker (fweisbec@...il.com) wrote:
> > On Mon, Jul 20, 2009 at 01:27:48PM -0400, Mathieu Desnoyers wrote:
> [...]
> > 
> > > Maybe we should think of a more flexible breakpoint type mapping too,
> > > e.g.:
> > > 
> > >  monitor _strictly_ execute operation on address 0x...
> > >    -> would fail if the architecture does not support execution access
> > >       monitoring
> > >  monitor (at least) execute operations on address 0x...
> > >    -> would be allowed to use a more general monitor (e.g. RWX) if the
> > >       architecture does not support "execute only" monitor.
> > > 
> > > (same for read and write)
> > > 
> > > Mathieu
> > 
> > 
> > Well, I'm not sure the problem mostly resides in the hardware implementation
> > of strict exec breakpoint types. But I guess your point is not limiting to
> > that. Yeah for example, x86 doesn't support read-only breakpoints.
> 
> Exactly. I used "execute" only as an example, but in the end, we could
> end up with a list looking like:
> 
> HW_WATCH_R	(1 << 0)
> HW_WATCH_NOT_R	(1 << 0)
> HW_WATCH_W	(1 << 1)
> HW_WATCH_NOT_W	(1 << 1)
> HW_WATCH_X	(1 << 2)
> HW_WATCH_NOT_X	(1 << 2)
> 
> So for instance, flags :
> 
> HW_WATCH_R|HW_WATCH_NOT_W|HW_WATCH_NOT_X
> 
> would specify that the architecture has to support a "read" watchpoint
> which does not trigger on write nor execute.
> 
> HW_WATCH_R
> 
> would specify that the architecture _must_ support "read" watchpoint,
> and we don't care about W or X.
> 
> HW_WATCH_R|HW_WATCH_W|HW_WATCH_X
> 
> Would ask for watching rwx on an address. The architecture would have to
> support all those three.
> 
> Some combinations would be invalid (e.g. HW_WATCH_R|HW_WATCH_NOT_R).
> 
> There might be better ways to express this, but at this it should show
> my point a bit more clearly.
> 
> Mathieu


Ok, I see your point.
Before making a choice to generically express the modes supported by an arch,
I need a good global view of what is supported by most archs,
thing that I lack and which I should look at quickly :)

Thanks.

 
> 
> > But I guess that can be simulated using software artifacts, for example using
> > READ-WRITE breakpoints + the x86 decoder API, recently submitted by Masami,
> > to find the nature of the current instruction.
> > 
> > Anyway, your point is indeed important: return common error values for unsupported
> > breakpoint operations.
> > 
> > Thanks.
> > 
> 
> -- 
> Mathieu Desnoyers
> OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68

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