Re: [PATCH 5/6] hw-breakpoints: Arbitrate access to pmu following registers constraints

From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>, Ingo Molnar <mingo@elte.hu>,
	LKML <linux-kernel@vger.kernel.org>,
	Prasad <prasad@linux.vnet.ibm.com>,
	Alan Stern <stern@rowland.harvard.edu>,
	Peter Zijlstra <peterz@infradead.org>,
	Arnaldo Carvalho de Melo <acme@redhat.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Jan Kiszka <jan.kiszka@web.de>, Jiri Slaby <jirislaby@gmail.com>,
	Li Zefan <lizf@cn.fujitsu.com>, Avi Kivity <avi@redhat.com>,
	Mike Galbraith <efault@gmx.de>,
	Masami Hiramatsu <mhiramat@redhat.com>,
	Paul Mundt <lethal@linux-sh.org>
Subject: Re: [PATCH 5/6] hw-breakpoints: Arbitrate access to pmu following registers constraints
Date: Fri, 13 Nov 2009 07:00:38 +1100	[thread overview]
Message-ID: <1258056038.2140.354.camel@pasglop> (raw)
In-Reply-To: <20091112155413.GE5237@nowhere>

On Thu, 2009-11-12 at 16:54 +0100, Frederic Weisbecker wrote:
> >  - On one embedded core at least we have a case where the core has 4
> > threads, but the data (4) and instruction (2) breakpoint registers are
> > shared. The 'enable' bits are split so a given data breakpoint can be
> > enabled only on some HW threads but that's about it.
> > 
> > I'm not sure if there's a realistic way to handle the later constraint
> > though other than just not allowing use of the HW breakpoint function on
> > those cores at all.
> > 
> > Ben.
> 
> 
> Yeah this latter one is tricky. Not sure how to handle it either.
> How are these hw-threads considered by the kernel core? As different
> cpu? 

Yes.

So it basically looks like you have 4 data and 2 HW instruction breakpoint
registers shared by 4 CPUs in a group :-)

Cheers,
Ben.