linux-kernel - Re: [RFC][PATCH] perf, intel: Don't touch MSR_IA32

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Message-ID: <1347471446.15764.67.camel@twins>
Date:	Wed, 12 Sep 2012 19:37:26 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Oleg Nesterov <oleg@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [RFC][PATCH] perf, intel: Don't touch MSR_IA32_DEBUGCTLMSR from
 NMI context

On Wed, 2012-09-12 at 18:42 +0200, Stephane Eranian wrote:
> We use FREEZE_LBR_ON_PMI to sync LBR data with counter overflows.
> That means, LBR is already frozen by the time we get to the handler. But
> that means we need to re-enable LBR when we leave the handler. I don't
> think EOI is going to magically re-enable it. So we need to touch DEBUGCTL
> in the irq handler. 

Ah, so I do think EIO will re-enable LBR, also the handler is wrapped in
x86_pmu::{dis,en}able_all() which does end up calling
intel_pmu_lbr_{dis,en}able_all(). However that leaves the MSR in the
exact same state on exit as it was on enter, so that's not a problem for
the: read-modify-write change.

Its just that the x86_pmu::disable() for the throttle could leave the
MSR in a different state on exit than it entered with. This patch avoids
that.
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