Re: [PATCH 1/2] perf/x86/intel/pt: Fail event scheduling on conflict with VMX

[Alexander Shishkin]

Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:

> On Tue, Feb 14, 2017 at 03:24:15PM +0200, Alexander Shishkin wrote:
>> At the moment, if VMX operation prevents PT tracing, the PMU will
>> silently return success to the event scheduling code, which will
>> track its 'on' time, etc. Instead, report failure so that perf
>> core knows this event is not actually on.
>> 
>> Signed-off-by: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
>> Reported-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
>> Fixes: 1c5ac21a0e ("perf/x86/intel/pt: Don't die on VMXON")
>> ---
>>  arch/x86/events/intel/pt.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/arch/x86/events/intel/pt.c b/arch/x86/events/intel/pt.c
>> index d92a60ef08..9372fa4549 100644
>> --- a/arch/x86/events/intel/pt.c
>> +++ b/arch/x86/events/intel/pt.c
>> @@ -1335,7 +1335,7 @@ static void pt_event_start(struct perf_event *event, int mode)
>>  	struct pt_buffer *buf;
>>  
>>  	if (READ_ONCE(pt->vmx_on))
>> -		return;
>> +		goto fail_stop;
>>  
>>  	buf = perf_aux_output_begin(&pt->handle, event);
>>  	if (!buf)
>
> I'm not getting it; how does this matter to the time tracking in
> event_sched_in() / event_sched_out() ?
>
> That looks at event->state == PERF_EVENT_STATE*
>
> This goto affects event->hw.state == PERF_HES_
>
> The core assumes ->start() will _NOT_ fail.

This is called by pmu::add(), which checks hw.state afterwards and if it
finds HES_STOPPED, it returns an error, which event_sched_in() captures
and keeps the event in INACTIVE state. Should I add a comment about it?

Regards,
--
Alex