Re: [PATCH v8 12/14] x86: perf: intel_pt: Intel PT PMU driver

[Peter Zijlstra]

On Tue, Jan 13, 2015 at 05:09:58PM +0200, Alexander Shishkin wrote:
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:
> 
> > On Fri, Nov 14, 2014 at 03:43:45PM +0200, Alexander Shishkin wrote:
> >> +static void pt_event_stop(struct perf_event *event, int mode)
> >> +{
> >> +	struct pt *pt = this_cpu_ptr(&pt_ctx);
> >> +
> >> +	ACCESS_ONCE(pt->handle_nmi) = 0;
> >
> > Why is this needed? Will the hardware still generate interrupts if you
> > stop the PT thing?
> 
> Actually, it turns out that the interrupt condition can race with the
> wrmsr that disables tracing and the PT bit remains set in the global
> status register, causing the next PMI to go into the PT PMI handler,
> which then may decide to re-enable the counter that should be disabled.

OK, please add this information in a suitably placed comment.

Thanks!
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