linux-kernel - One question about perf

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Message-ID: <20090527071736.GA26774@ywang-moblin2.bj.intel.com>
Date:	Wed, 27 May 2009 15:17:36 +0800
From:	Yong Wang <yong.y.wang@...ux.intel.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>
Cc:	linux-kernel@...r.kernel.org
Subject: One question about perf_counter on IA

I noticed that the LVT error register is masked and then unmasked during
perf_counter initialization on IA in arch/x86/kernel/cpu/perf_counter.c

void perf_counters_lapic_init(int nmi)
{
	u32 apic_val;

	if (!x86_pmu_initialized())
		return;

	/*
	 * Enable the performance counter vector in the APIC LVT:
	 */
	apic_val = apic_read(APIC_LVTERR);

	apic_write(APIC_LVTERR, apic_val | APIC_LVT_MASKED);
	if (nmi)
		apic_write(APIC_LVTPC, APIC_DM_NMI);
	else
		apic_write(APIC_LVTPC, LOCAL_PERF_VECTOR);
	apic_write(APIC_LVTERR, apic_val);
}

Why is this needed? Is this documented somewhere in the spec? I tried
commenting out the masking and unmasking operations and I did not any
error interrupt generated.

Thanks
-Yong
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