linux-kernel - Re: [Patch 0/1] HW-BKPT: Allow per-cpu kernel-space Hardware Breakpoint requests

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Message-ID: <20090825203307.GA5834@in.ibm.com>
Date:	Wed, 26 Aug 2009 02:03:07 +0530
From:	"K.Prasad" <prasad@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Alan Stern <stern@...land.harvard.edu>
Subject: Re: [Patch 0/1] HW-BKPT: Allow per-cpu kernel-space Hardware
	Breakpoint requests

On Thu, Aug 20, 2009 at 10:57:19PM +0530, K.Prasad wrote:
> On Wed, Aug 19, 2009 at 07:33:00PM +0200, Frederic Weisbecker wrote:
> > On Wed, Aug 19, 2009 at 09:41:19PM +0530, K.Prasad wrote:
> > > On Mon, Aug 17, 2009 at 06:16:41PM +0530, K.Prasad wrote:
> > > > Hi All,
> > > > 	Please find a patch that enables kernel-space breakpoints to be
> > > > requested for a subset of the available CPUs in the system. This allows
> > > > per-CPU breakpoints and comes with the associated benefit of reduced
> > > > overhead during (un)registration.
> > > > 
> > > > This enhancement allows exploitation of hardware breakpoint registers by
> > > > 'perf' which produces a CPU-wise information.
> > > > 
> [edited]
> > > 
> > > Hi Frederic,
> > > 	Do you find these patches, that provide the ability to restrict
> > > kernel-space breakpoints to any given subset of CPUs, to bring the
> > > requisite features for exploitation of hw-bkpt by 'perf tools'?
> > > 
> > > Also of interest would be the reduced overhead associated with
> > > (un)register_kernel_hw_breakpoint() operations (no IPI in case of
> > > single-CPU breakpoint request).
> > > 
[edited]
> > - having a callback that quickly swap two breakpoints in order to support
> >   the hardware register multiplexing. I guess the pmu object would just need
> >   to call it when the multiplexing is decided.
> > 
> > 
> 
> Are you suggesting something like a modify_kernel_hw_breakpoint() that
> can quickly change a breakpoint address/characteristics?
> 
> That's quite doable...it requires a quick validation through
> arch_validate_hwbkpt_settings() and the requisite IPIs (depending on
> what the new cpumask is).
> 
> I will send a patch to that effect soon.
>

Hi Frederic,
	I just sent a patchset that adds the ability to specify per-cpu
kernel-space breakpoints + a (relatively) lightweight function to modify
the characteristics of a kernel-space breakpoint that can be used to
swap between two breakpoint requests.

Please pull them into -tip tree if you find them mature and ready.

With these new feature additions, I see the HW-Breakpoint infrastructure
code ready to meet the needs for exploitation by perf-tools and I presume
you would restart your effort on the same?

Thanks,
K.Prasad

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