Re: [PATCH v5 12/20] x86: perf: intel_pt: Intel PT PMU driver

[Peter Zijlstra]

On Fri, Oct 24, 2014 at 10:49:33AM +0300, Alexander Shishkin wrote:
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:
> 
> > On Mon, Oct 13, 2014 at 04:45:40PM +0300, Alexander Shishkin wrote:
> >> +++ b/arch/x86/kernel/cpu/perf_event_intel.c
> >> @@ -1528,6 +1528,14 @@ again:
> >>  	}
> >>  
> >>  	/*
> >> +	 * Intel PT
> >> +	 */
> >> +	if (__test_and_clear_bit(55, (unsigned long *)&status)) {
> >> +		handled++;
> >> +		intel_pt_interrupt();
> >> +	}
> >> +
> >
> > How does the PT interrupt interact with the regular PMI? In particular
> > does it respect stuff like FREEZE_ON_PMI etc?
> 
> It ignores the FREEZE_ON_PMI bit. I stop it by hand inside the PMI
> handler, so you can see parts of the handler in the trace if you're
> tracing the kernel.

Urgh, horrid that. Routing something to the same interrupt, sharing
status registers but not observing the same semantics for the interrupt
is a massive fail.

IIRC Andi was planning to start using FREEZE_ON_PMI to avoid the MSR
writes in intel_pmu_{disable,enable}_all(), this interrupt not actually
respecting that makes that non-trivial.

We already use FREEZE_ON_PMI for LBR, but for now PT and LBR are
mutually exclusive so that's not a problem, if we ever get those working
together this needs to get fixed.
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