linux-kernel - Re: [PATCH 2/3] x86, perf: Add a separate Arch Perfmon v4 PMI handler

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Date:   Tue, 7 Aug 2018 19:31:37 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Liang, Kan" <kan.liang@...ux.intel.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, acme@...nel.org,
        linux-kernel@...r.kernel.org, eranian@...gle.com,
        ak@...ux.intel.com, alexander.shishkin@...ux.intel.com
Subject: Re: [PATCH 2/3] x86, perf: Add a separate Arch Perfmon v4 PMI handler

On Tue, Aug 07, 2018 at 11:29:54AM -0400, Liang, Kan wrote:
> On 8/6/2018 2:35 PM, Peter Zijlstra wrote:
> > On Mon, Aug 06, 2018 at 10:23:42AM -0700, kan.liang@...ux.intel.com wrote:
> > > @@ -2044,6 +2056,14 @@ static void intel_pmu_disable_event(struct perf_event *event)
> > >   	if (unlikely(event->attr.precise_ip))
> > >   		intel_pmu_pebs_disable(event);
> > > +	/*
> > > +	 * We could disable freezing here, but doesn't hurt if it's on.
> > > +	 * perf remembers the state, and someone else will likely
> > > +	 * reinitialize.
> > > +	 *
> > > +	 * This avoids an extra MSR write in many situations.
> > > +	 */
> > > +
> > >   	if (unlikely(hwc->config_base == MSR_ARCH_PERFMON_FIXED_CTR_CTRL)) {
> > >   		intel_pmu_disable_fixed(hwc);
> > >   		return;
> > > @@ -2119,6 +2139,11 @@ static void intel_pmu_enable_event(struct perf_event *event)
> > >   	if (event->attr.exclude_guest)
> > >   		cpuc->intel_ctrl_host_mask |= (1ull << hwc->idx);
> > > +	if (x86_pmu.counter_freezing && !cpuc->frozen_enabled) {
> > > +		enable_counter_freeze();
> > > +		cpuc->frozen_enabled = 1;
> > > +	}
> > > +
> > >   	if (unlikely(event_is_checkpointed(event)))
> > >   		cpuc->intel_cp_status |= (1ull << hwc->idx);
> > 
> > Why here? That doesn't really make sense; should this not be in
> > intel_pmu_cpu_starting() or something?
> 
> 
> For Goldmont Plus, the counter freezing feature can be re-enabled at
> run-time by loading a newer microcode.
> We need to check the x86_pmu.counter_freezing every time.

Blergh, just don't go there. If we start with the wrong ucode, leave it
disabled.

We do that for most ucode stuff.