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Date: Thu, 3 Sep 2009 23:58:10 +0530
From: "K.Prasad" <prasad@linux.vnet.ibm.com>
To: Frederic Weisbecker <fweisbec@gmail.com>, Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <peterz@infradead.org>, LKML <linux-kernel@vger.kernel.org>,
       Lai Jiangshan <laijs@cn.fujitsu.com>,
       Steven Rostedt <rostedt@goodmis.org>,
       Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
       Alan Stern <stern@rowland.harvard.edu>,
       Paul Mackerras <paulus@samba.org>, David Gibson <dwg@au1.ibm.com>
Subject: Re: [Patch 0/1] HW-BKPT: Allow per-cpu kernel-space Hardware
	Breakpoint requests
Message-ID: <20090903182810.GA3952@in.ibm.com>
Reply-To: prasad@linux.vnet.ibm.com
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On Wed, Sep 02, 2009 at 01:51:33AM +0200, Frederic Weisbecker wrote:
> On Tue, Sep 01, 2009 at 12:08:45PM +0530, K.Prasad wrote:
> > On Sat, Aug 29, 2009 at 03:41:07PM +0200, Ingo Molnar wrote:
> > > 
> > > * K.Prasad <prasad@linux.vnet.ibm.com> wrote:
> > > 
> > > > I am not sure if pmus can handle, (or want to handle) all the 
> > > > intricacies involved with the hw-breakpoint layer [...]
> > > 
> > > Which are those intricacies? It's all rather straightforward 
> > > register scheduling and reservation stuff - which perfcounters 
> > > already solves in a very rich way.
> > > 
> > > 	Ingo
> > 
[edited]
> > And post integration, in-kernel users like ptrace, kgdb* and xmon*
> > which hitherto have interacted directly with the debug registers
> > (through set_debugreg()/set_dabr()) should route their requests through the
> > perf-layer. It is difficult to imagine ptrace's idempotent requests
> > (through ptrace_<get><set>_debugreg()) having to pass through perf-layer
> > (and becoming dependant on CONFIG_PERF_COUNTERS), not to mention the
> > tricks required to synchronise signal generation timing with exception
> > behaviour (especially on PPC64).
> > * - Not converted to use hw-breakpoint layer yet
> 
> 
> Actually, I see the perf layer here as a middle man between
> 
> - the very hardware stuff (dr[0-467]) handling, reading, writing, updating
> - the core API (register_kernel_breakpoint(), register_user_breakpoint() etc..)
> 
> And this middle man can handle so much things on its own that the two above
> gets utterly shrinked.
> 
> Also the ptrace thing is tricky in itself, and that can't be helped easily.
> Because of the direct writing to debug registers done by POKE_USR,
> whatever the current breakpoint API with or without perf integration, we still
> need subterfuges to carry it.
>

The reverse-dependancy this would create over perf (CONFIG_PERF) for the
hw-breakpoint layer is an undesirable side-effect, and gives rise to
atleast two immediate questions:

- Handling of requests for hw-breakpoint from users like ptrace when
  CONFIG_PERF is not turned on
- Managing 'register scheduling and reservation' on architectures where
  perf layer isn't ported. An inefficient way of handling this would be
  to retain the existing register allocation code of hw-breakpoint for
  such architectures - thereby artificially imposing arch-specific code
  into generic stuff.

A solution here would be to detach parts of perf layer's code that
handle register scheduling and reservation (which I learn are in
kernel/perf_counter.c) into a separate entity (outside the ambit of
CONFIG_PERF) that can serve the needs of both hw-breakpoint and perf
thereby eliminating the two issues enumerated above.

The tight coupling between the functions that perform register
scheduling (in kernel/perf_counter.c) and perf's data structures is quite
apparent and does suggest non-trivial amount of effort to detach them
into a layer of its own.

However this might be quite necessary in order to balance between a
desire to re-use the 'register scheduling and reservation' code of
perf-layer while not running into issues as above.

This, along with the framework (described in the previous mail) to retain
the hw-breakpoint's APIs + code interacting with debug registers
(including exception handling) would be a good compromise.

Thanks,
K.Prasad

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