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

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Message-ID: <20140708092513.GB6758@twins.programming.kicks-ass.net>
Date:	Tue, 8 Jul 2014 11:25:13 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	kan.liang@...el.com
Cc:	andi@...stfloor.org, linux-kernel@...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).



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