linux-kernel - Re: [PATCH 3/5] hw-breakpoints: Rewrite the hw-breakpoints layer on top of perf counters

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Date:	Mon, 14 Sep 2009 22:58:35 +0530
From:	"K.Prasad" <prasad@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Alan Stern <stern@...land.harvard.edu>,
	Peter Zijlstra <peterz@...radead.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Jan Kiszka <jan.kiszka@...mens.com>,
	Jiri Slaby <jirislaby@...il.com>,
	Li Zefan <lizf@...fujitsu.com>, Avi Kivity <avi@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Mike Galbraith <efault@....de>,
	Masami Hiramatsu <mhiramat@...hat.com>
Subject: Re: [PATCH 3/5] hw-breakpoints: Rewrite the hw-breakpoints layer
	on top of perf counters

On Thu, Sep 10, 2009 at 10:29:25AM +0200, Frederic Weisbecker wrote:
> This patch rebase the implementation of the breakpoints API on top of
> perf counters instances.
> 
> The core breakpoint API has changed a bit:
> 
> - register_kernel_hw_breakpoint() now takes a cpu as a parameter. For
>   now it doesn't support all cpu wide breakpoints but this may be
>   implemented soon.
> 
> - unregister_kernel_hw_breakpoint() and unregister_user_hw_breakpoint()
>   have been unified in a single unregister_hw_breakpoint()
> 
> Each breakpoints now match a perf counter which now handles the
> register scheduling, thread/cpu attachment, etc..
> 
[edited]
> 
> -/*
> - * Load the debug registers during startup of a CPU.
> - */
> -void load_debug_registers(void)

It does not appear that perf handles CPUs that come up new (if else,
blame my understanding of find_get_context():-)) and hence post breakpoint
integration, the new CPUs wouldn't contain any breakpoint values (meant
for all CPUs). As mentioned in my previous mail, this could be
non-trivial lapse in debugging scenarios and even for users like
ksym_tracer in ftrace. Your patch would want to retain the
functionality of load_debug_registers().

Thanks,
K.Prasad

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