linux-kernel - Re: [PATCH 11/12] hw-breakpoints: ftrace plugin for kernel symbol tracing using HW Breakpoint interfaces

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Date:	Fri, 5 Jun 2009 13:44:35 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"K.Prasad" <prasad@...ux.vnet.ibm.com>
Cc:	David Daney <ddaney@...iumnetworks.com>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Alan Stern <stern@...land.harvard.edu>
Subject: Re: [PATCH 11/12] hw-breakpoints: ftrace plugin for kernel symbol
	tracing using HW Breakpoint interfaces

On Thu, Jun 04, 2009 at 09:15:00PM +0530, K.Prasad wrote:
> On Wed, Jun 03, 2009 at 02:38:12AM +0200, Frederic Weisbecker wrote:
> > On Tue, Jun 02, 2009 at 04:12:08PM -0700, David Daney wrote:
> > > Frederic Weisbecker wrote:
> > >> From: K.Prasad <prasad@...ux.vnet.ibm.com>
> 
> > > I hate to sound like a broken record, but could some one explain to me  
> > > again why it is a good idea to design a new API that requires processor  
> > > specific #ifdefs to be sprinkled all around generic kernel code?
> > >
> > > Back in:
> > > http://lkml.org/lkml/2008/12/4/329
> > > and
> > > http://lkml.org/lkml/2009/5/21/189
> > >
> > > I raised doubts about this hw-breakpoint thing being generic and the  
> > > responses made think that the processor specific portions would be  
> > > isolated in the processor specific parts of the kernel.  I now see that  
> > > I was wrong.
> > >
> > > When we add sparc, MIPS, ppc...  Support it would be nice to not have to  
> > > add all our own #ifdefs to this, but instead have a generic interface  
> > > that will not need changes.
> > >
> > > David Daney
> > 
> > I was discussing about it with Prasad few hours ago :)
> > 
> > The fact is that archs support the hardware breakpoints in
> > very different ways each.
> > Some of them support read breakpoint, others not (x86).
> > Some support addresses range, others (x86).
> > 
> > But still it would be nice to gather the most common
> > breakpoints operations through a real generic wrapper
> > that relies on arch specific implmentation in
> > background.
> > 
> > Such as setting very simple x/w/r breakpoints...
> > 
> > Well Prasad and Alan Stern could tell more about it,
> > I wait for their answer.
> > 
> > Anyway it's a fairly new Api that can still evolve.
> > The basis are set but can still be improved and more high level
> > and generic things can still be implemented.
> > 
> 
> I think this concern can be partially addressed, atleast as far as the
> breakpoint length is concerned. I've added my comments in the response
> to David Daney here: http://lkml.org/lkml/2009/6/4/303.
> 
> Hope that the changes proposed there is acceptable to the community.


Yeah, I think it's worth trying it.

Btw, I see there is no easy way to implement read breapoints in x86.
I guess the only solution would be to use the instruction decoder sent
recently by Masami for the kprobe tracer, coupled with RW breakpoints.

 
> Thanks,
> K.Prasad
> 

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