linux-kernel - RE: [PATCH V3 1/2] perf ignore LBR and offcore

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Date:	Tue, 8 Jul 2014 14:20:48 +0000
From:	"Liang, Kan" <kan.liang@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	"andi@...stfloor.org" <andi@...stfloor.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: RE: [PATCH V3 1/2] perf ignore LBR and offcore_rsp.



> 
> On Mon, Jul 07, 2014 at 06:34:25AM -0700, kan.liang@...el.com wrote:
> > +	/*
> > +	 * Access LBR MSR may cause #GP under certain circumstances.
> > +	 * E.g. KVM doesn't support LBR MSR
> > +	 * Check all LBT MSR here.
> > +	 * Disable LBR access if any LBR MSRs can not be accessed.
> > +	 */
> > +	if (x86_pmu.lbr_nr) {
> > +		x86_pmu.lbr_msr_access = check_msr(x86_pmu.lbr_tos);
> > +		for (i = 0; i < x86_pmu.lbr_nr; i++) {
> > +			x86_pmu.lbr_msr_access &=
> > +				check_msr(x86_pmu.lbr_from + i);
> > +			x86_pmu.lbr_msr_access &=
> > +				check_msr(x86_pmu.lbr_to + i);
> > +		}
> > +	}
> 
> So I was going to refer you to an email I send earlier telling you that this was
> wrong, but then found it got stuck in a mailqueue on my laptop :-(
> 
> In any case its wrong; just clear lbr_nr and kill lbr_msr_access. We already
> check lbr_nr in all the right places and you added lbr_msr_access to far too
> few of them (also the fact that every place you check lbr_msr_access already
> had an lbr_nr test should've been a clue).

OK. If lbr_nr is well checked at runtime, I will simply set lbr_nr to 0 here (once check_msr failed).

Thanks,
Kan 

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