linux-kernel - Re: [PATCH 1/1] perf/x86: filter branches for PEBS event

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Date:	Fri, 27 Mar 2015 13:10:52 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	"Liang, Kan" <kan.liang@...el.com>,
	Andi Kleen <andi@...stfloor.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] perf/x86: filter branches for PEBS event

On Thu, Mar 26, 2015 at 12:40:14PM -0700, Stephane Eranian wrote:
> You are addressing one of the problems of this routine. But I think
> there is a more serious issue which is not addressed here. The
> intel_shared_regs_constraints() assumes that the associated event is
> necessarily unconstrained:
> 
> __intel_shared_reg_get_constraints()
> {
>     struct event_constraint *c = &emptyconstraint;
>    ...
> }

emptyconstraint != unconstrained.

Note how that function only returns emptyconstraint if its rejecting the
event, otherwise it returns NULL such that we continue calling
x86_get_event_constraint().

> This is true for offcore_response, but for LBR this may not always be the case.
> I may want to use LBR on the L1D_PEND_MISS event and it would need to
> be on counter 2.

> But I believe that the current code could place it on counter 0 simply
> because you return if shared_reg_get_constraint() is successful, but
> it looks only at the LBR constraint not the event constraint. I think
> in the presence of LBR, you always need to call share_get_reg() and
> x86_get_event_constraint().

Which, I think it does.
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