linux-kernel - Re: [PATCH 4/6] hw-breakpoints: Rewrite the hw-breakpoints layer on top of perf events

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Message-ID: <19186.45127.262567.839766@cargo.ozlabs.ibm.com>
Date:	Thu, 5 Nov 2009 22:00:23 +1100
From:	Paul Mackerras <paulus@...ba.org>
To:	prasad@...ux.vnet.ibm.com
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Alan Stern <stern@...land.harvard.edu>,
	Peter Zijlstra <peterz@...radead.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Jan Kiszka <jan.kiszka@....de>,
	Jiri Slaby <jirislaby@...il.com>,
	Li Zefan <lizf@...fujitsu.com>, Avi Kivity <avi@...hat.com>,
	Mike Galbraith <efault@....de>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Paul Mundt <lethal@...ux-sh.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 4/6] hw-breakpoints: Rewrite the hw-breakpoints layer
	on top of perf events

K.Prasad writes:

> On Thu, Nov 05, 2009 at 10:59:44AM +1100, Paul Mackerras wrote:
> > What I haven't managed to understand yet is how you provide reliable
> > breakpoints for debugging purposes.  If I'm debugging a program and I
> > have set a breakpoint, I'll be very unhappy if the breakpoint should
> > trigger but doesn't because the perf_event infrastructure has decided
> > it can't schedule that breakpoint in.  If the breakpoint isn't going
> > to work then I want to know that at the time that I set it.
> >
> 
> The hw-breakpoint layer avoids such issues by not over-committing debug
> registers i.e. 'n' number of debug registers are reserved for user-space
> where n = max(no. of breakpoints requested by any given process/thread).
> And book-keeping certainly helps here too.

Yes, I missed the fact that there was still a little bit of the
hw-breakpoints layer underneath the perf_event layer.  I'm not sure
that what Frederic has now will suit those Power CPUs that have both
an IABR and DABR, but we can fix that later.

Paul.
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