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

[Alexander Shishkin]

Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:

> 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.

I can't pretend to understand the logic behind this either.

> 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.

Agreed.

Regards,
--
Alex
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