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Date: Mon, 13 Dec 2010 17:51:06 +0100
From: Andi Kleen <andi@firstfloor.org>
To: Stephane Eranian <eranian@google.com>
Cc: Lin Ming <ming.m.lin@intel.com>, Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Andi Kleen <andi@firstfloor.org>, Ingo Molnar <mingo@elte.hu>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Arjan van de Ven <arjan@infradead.org>,
        lkml <linux-kernel@vger.kernel.org>
Subject: Re: [RFC PATCH 2/3 v3] perf: Implement Nehalem uncore pmu
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> If think there you want something like:
> 
> if (cpu == cpumask_first(topology_core_siblings(cpu)) {
>       rdmsrl(MSR_IA32_DEBUGCTLMSR, val);
>      wrmsrl(MSR_IA32_DEBUGCTLMSR, val | DEBUGCTLMSR_ENABLE_UNCORE_PMI);
> }

You have to be careful with hotplug. The code would need to be rerun
on hotplug. Otherwise the interrupt may end up on a downed CPU.
-Andi