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Date:   Thu, 21 Mar 2019 19:20:11 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Stephane Eranian <eranian@google.com>,
        Ingo Molnar <mingo@kernel.org>, Jiri Olsa <jolsa@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>, tonyj@suse.com,
        nelson.dsouza@intel.com
Subject: Re: [PATCH 1/8] perf/x86/intel: Fix memory corruption
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On Thu, Mar 21, 2019 at 06:17:01PM +0100, Thomas Gleixner wrote:
> On Thu, 21 Mar 2019, Peter Zijlstra wrote:
> > On Thu, Mar 21, 2019 at 05:45:41PM +0100, Thomas Gleixner wrote:
> > > On Thu, 21 Mar 2019, Peter Zijlstra wrote:
> > > > Subject: perf/x86/intel: Initialize TFA MSR
> > > > 
> > > > Stephane reported that we don't initialize the TFA MSR, which could lead
> > > > to trouble if the RESET value is not 0 or on kexec.
> > > 
> > > That sentence doesn't parse.
> > > 
> > >   Stephane reported that the TFA MSR is not initialized by the kernel, but
> > >   the TFA bit could set by firmware or as a leftover from a kexec, which
> > >   makes the state inconsistent.
> > > 
> > > > Reported-by: Stephane Eranian <eranian@google.com>
> > > > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > > > ---
> > > >  arch/x86/events/intel/core.c | 6 ++++++
> > > >  1 file changed, 6 insertions(+)
> > > > 
> > > > diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
> > > > index 8baa441d8000..2d3caf2d1384 100644
> > > > --- a/arch/x86/events/intel/core.c
> > > > +++ b/arch/x86/events/intel/core.c
> > > > @@ -3575,6 +3575,12 @@ static void intel_pmu_cpu_starting(int cpu)
> > > >  
> > > >  	cpuc->lbr_sel = NULL;
> > > >  
> > > > +	if (x86_pmu.flags & PMU_FL_TFA) {
> > > > +		WARN_ON_ONCE(cpuc->tfa_shadow);
> > > 
> > > Hmm. I wouldn't warn here as this is a legit state for kexec/kdump and I
> > > don't think we can figure out whether this comes directly from the
> > > firmware.
> > 
> > Even on kexec, the cpuc will be freshly allocated and zerod I think. We
> > only inherit hardware state, not software state.
> 
> Ouch, misread the patch of course ... What about cpu hotplug? Does that
> free/alloc or reuse?

re-use I think, but since we kill all events before taking the CPU down,
it _should_ have cleared the MSR while doing that.

Might be best to have someone test this.. someone with ucode and a
machine etc.. :-)