Re: [PATCH 2/6] perf/x86/intel/pt: Stop tracing if VMXON has not yet happened

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Nov 02, 2026 at 09:40:35AM +0200, Adrian Hunter wrote:

>  arch/x86/events/intel/pt.c | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/events/intel/pt.c b/arch/x86/events/intel/pt.c
> index 49349afee611..36bf039668d7 100644
> --- a/arch/x86/events/intel/pt.c
> +++ b/arch/x86/events/intel/pt.c
> @@ -567,6 +567,15 @@ static void pt_config(struct perf_event *event)
>  	pt_config_enable(event);
>  }
>  
> +static bool pt_trace_enabled(void)
> +{
> +	u64 ctl;
> +
> +	rdmsrq(MSR_IA32_RTIT_CTL, ctl);
> +
> +	return ctl & RTIT_CTL_TRACEEN;
> +}
> +
>  static void pt_config_stop(struct perf_event *event)
>  {
>  	struct pt *pt = this_cpu_ptr(&pt_ctx);
> @@ -577,7 +586,12 @@ static void pt_config_stop(struct perf_event *event)
>  		return;
>  
>  	ctl &= ~RTIT_CTL_TRACEEN;
> -	if (!READ_ONCE(pt->vmx_on))
> +	/*
> +	 * vmx_on is set before VMXON, so tracing may not have stopped yet.
> +	 * RTIT_CTL can be written until VMXON, and after VMXON TraceEn is
> +	 * clear, so still write RTIT_CTL if TraceEn is set.
> +	 */
> +	if (!READ_ONCE(pt->vmx_on) || pt_trace_enabled())
>  		wrmsrq(MSR_IA32_RTIT_CTL, ctl);
>  
>  	WRITE_ONCE(event->hw.aux_config, ctl);

It seems rather unfortunate to issue a RDMSR for state we fully control.
Surely we can track this?