[perfmon] Re: perfmon2 TODO list (4/4)

[Stephane Eranian]

Will,

On Mon, Apr 17, 2006 at 09:56:57AM -0400, William Cohen wrote:
> Stephane Eranian wrote:
> >Frank,
> >
> >On Thu, Apr 13, 2006 at 06:22:51PM -0400, Frank Ch. Eigler wrote:
> >
> >>>I have another question related maybe more to kprobes and how the
> >>>intercept is done: breakpoints, code rewriting. If you use
> >>>breakpoints, then I wonder how this works in SMP machines. Do you
> >>>intervene on each CPU?
> >>
> >>That's right: as each CPU trips across a breakpoint, they are made to
> >>run our handler, then single-step across the original instruction,
> >>then resume.  It's a multi-step process described in kprobes
> >>documentation.  From systemtap's point of view, it's a black box.
> >>
> >
> >So you are saying that kprobes takes care of programming the debug
> >registers on all CPUs if necessary.
> >
> 
> Kprobe uses breakpoint instructions, so the breakpoint registers on the 
> processor are not currently being used. Thus, a breakpoint instruction 
> is placed at the location where the probe is desired and the processor's 
> debugging registers are not touch by kprobes.
> 
> There has been some discussion for SystemTap producing probes that use 
> the processors debug hardware to watch for accesses to specific memory 
> locations.
> 
So from what you are saying neither kprobes nor systemap uses IPI for any setup/tear
down at this point. Is that right?

-- 

-Stephane