linux-kernel - Re: [PATCH V4 1/2] perf ignore LBR and extra

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Date:	Wed, 9 Jul 2014 11:19:17 +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 V4 1/2] perf ignore LBR and extra_regs.

On Tue, Jul 08, 2014 at 09:49:40AM -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) {
> +		access = check_msr(x86_pmu.lbr_tos);
> +		for (i = 0; i < x86_pmu.lbr_nr; i++) {
> +			access &= check_msr(x86_pmu.lbr_from + i);
> +			access &= check_msr(x86_pmu.lbr_to + i);
> +		}
> +		if (!access)
> +			x86_pmu.lbr_nr = 0;
> +	}

You _could_ write that as:

	if (x86_pmu.lbr_nr) {
		if (!check_msr(x86_pmu.lbr_tos))
			x86_pmu.lbr_nr = 0;
		for (i = 0; i < x86_pmu.lbr_nr; i++) {
			if (!(check_msr(x86_pmu.lbr_from + i) &&
			      check_msr(x86_pmu.lbr_to + i)))
			      x86_pmu.lbr_nr = 0;
		}
	}

There is no point in checking any more MSRs after the first fail after
all.

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