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