linux-kernel - Re: HW breakpoints perf

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Date:	Tue, 19 Jan 2010 12:44:14 -0500
From:	"Frank Ch. Eigler" <fche@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Joshua Pincus <joshua.pincus@...il.com>,
	Andi Kleen <andi@...stfloor.org>,
	"K.Prasad" <prasad@...ux.vnet.ibm.com>, peterz@...radead.org,
	paulus@...ba.org, acme@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: HW breakpoints perf_events request

Hi -

On Tue, Jan 19, 2010 at 05:21:02PM +0100, Frederic Weisbecker wrote:
> [...]
> But I rather hope we can extend ptrace interface to handle such new
> needs instead (ie: having a more scalable breakpoint interface
> support by ptrace).

According to its maintainer, ptrace per se appears to be not well
suited for extensions that affect control flow, but maybe.
(http://www.mail-archive.com/utrace-devel@redhat.com/msg02276.html)

> > Another is using the gdbstub, extended with gdb watchpoint support (Z*
> > packets), which would tie into the hw-breakpoint system directly.
> > [...]

> Is this gdbstub an interface to utrace?
> This: http://lwn.net/Articles/364268/ ?

Yes, but I wouldn't think of it that way ("an interface to utrace").
Yes, it uses utrace, but that's an implementation detail.  To
userspace it presents gdb's existing wire protocol for debugging
processes.

> > > Do you plan a resubmission soon?
> > 
> > Utrace core has been resubmitted at the end of December
> > (http://lkml.org/lkml/2009/12/17/466), with no further comments
> > received.
> 
> Hmm, there has been deep review from Peter, IIRC.

I haven't seen any after that particular resubmission.  Rather, there
has been lots of discussion lately about *uprobes*, which is a
separate & optional process breakpoint management layer that happens
to use utrace and happens to be used by systemtap and the gdbstub.

- FChE
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