Re: [PATCH 4/6] hw-breakpoints: Rewrite the hw-breakpoints layeron top of perf events

From: Paul Mackerras
Date: Thu Nov 05 2009 - 06:04:11 EST

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.

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