linux-kernel - Re: [PATCH V2 3/3] perf/x86: Reset the dirty counter to prevent the leak for an RDPMC task

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Date:   Tue, 8 Sep 2020 17:58:40 +0200
From:   peterz@...radead.org
To:     kan.liang@...ux.intel.com
Cc:     mingo@...hat.com, acme@...nel.org, linux-kernel@...r.kernel.org,
        ak@...ux.intel.com, mark.rutland@....com, luto@...capital.net
Subject: Re: [PATCH V2 3/3] perf/x86: Reset the dirty counter to prevent the
 leak for an RDPMC task

On Mon, Sep 07, 2020 at 06:01:15PM +0200, peterz@...radead.org wrote:
> On Fri, Aug 21, 2020 at 12:57:54PM -0700, kan.liang@...ux.intel.com wrote:
> > diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
> > index 0f3d01562ded..fa08d810dcd2 100644
> > --- a/arch/x86/events/core.c
> > +++ b/arch/x86/events/core.c
> > @@ -1440,7 +1440,10 @@ static void x86_pmu_start(struct perf_event *event, int flags)
> >  
> >  	cpuc->events[idx] = event;
> >  	__set_bit(idx, cpuc->active_mask);
> > -	__set_bit(idx, cpuc->running);
> > +	/* The cpuc->running is only used by the P4 PMU */
> > +	if (!cpu_has(&boot_cpu_data, X86_FEATURE_ARCH_PERFMON) &&
> > +	    (boot_cpu_data.x86 == 0xf))
> > +		__set_bit(idx, cpuc->running);
> >  	x86_pmu.enable(event);
> >  	perf_event_update_userpage(event);
> >  }
> 
> Yuck! Use a static_branch() or something. This is a gnarly nest of code
> that runs 99.9% of the time to conclude a negative. IOW a complete waste
> of cycles for everybody not running a P4 space heater.

Better still, move it into p4_pmu_enable_event().