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

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Message-ID: <20170214135620.GN6500@twins.programming.kicks-ass.net>
Date:   Tue, 14 Feb 2017 14:56:20 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        vince@...ter.net, eranian@...gle.com,
        Arnaldo Carvalho de Melo <acme@...radead.org>,
        Borislav Petkov <bp@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 1/2] perf/x86/intel/pt: Fail event scheduling on conflict
 with VMX

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@...ux.intel.com>
> Reported-by: Andi Kleen <ak@...ux.intel.com>
> 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.