Re: [PATCH 5/7] hw-breakpoints: Rewrite the hw-breakpoints layer on top of perf events

From: "K.Prasad" <prasad@linux.vnet.ibm.com>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>, LKML <linux-kernel@vger.kernel.org>,
	Li Zefan <lizf@cn.fujitsu.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>,
	Avi Kivity <avi@redhat.com>, Paul Mackerras <paulus@samba.org>,
	Mike Galbraith <efault@gmx.de>,
	Masami Hiramatsu <mhiramat@redhat.com>,
	Paul Mundt <lethal@linux-sh.org>,
	Arjan van de Ven <arjan@linux.intel.com>
Subject: Re: [PATCH 5/7] hw-breakpoints: Rewrite the hw-breakpoints layer on top of perf events
Date: Mon, 16 Nov 2009 19:58:11 +0530	[thread overview]
Message-ID: <20091116142811.GA24341@in.ibm.com> (raw)
In-Reply-To: <20091112151947.GC5237@nowhere>

On Thu, Nov 12, 2009 at 04:19:52PM +0100, Frederic Weisbecker wrote:
> On Sun, Nov 08, 2009 at 11:01:07PM +0530, K.Prasad wrote:
> > 
> > A few more observations....
> > 
> > int reserve_bp_slot(struct perf_event *bp)
> > {
> > ...
> > ....
> > 	if (!bp->attr.pinned) {
> > 		/*
> > 		 * If there are already flexible counters here,
> > 		 * there is at least one slot reserved for all
> > 		 * of them. Just join the party.
> > 		 *
> > 		 * Otherwise, check there is at least one free slot
> > 		 */
> > 		if (!slots.flexible && slots.pinned == HBP_NUM) {
> > 			ret = -ENOSPC;
> > 			goto end;
> > 		}
> > 
> > 	/* Flexible counters need to keep at least one slot */
> > 	} else if (slots.pinned + (!!slots.flexible) == HBP_NUM) {
> > 		ret = -ENOSPC;
> > 		goto end;
> > 	}
> > ..
> > ...
> > }
> > 
> > It appears that you're reserving one slot for the non-pinned breakpoint
> > requests, which I'm afraid wouldn't play well with PPC64 (having one
> > DABR).
> 
> I don't understand what you mean. PPC64 has only one DABR, or...?
> 

Yes, PPC64 has just one DABR. And so this scheme will allow the first
request (be it 'pinned' or 'unpinned') to use the debug register? Sounds
fine.

Thanks,
K.Prasad