Re: [PATCH V2 02/11] perf/x86: Add support for TSC as a perf event clock

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Feb 14, 2022 at 01:09:05PM +0200, Adrian Hunter wrote:
> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> index 82858b697c05..e8617efd552b 100644
> --- a/include/uapi/linux/perf_event.h
> +++ b/include/uapi/linux/perf_event.h
> @@ -290,6 +290,15 @@ enum {
>  	PERF_TXN_ABORT_SHIFT = 32,
>  };
>  
> +/*
> + * If supported, clockid value to select an architecture dependent hardware
> + * clock. Note this means the unit of time is ticks not nanoseconds.
> + * Requires ns_clockid to be set in addition to use_clockid.
> + * On x86, this clock is provided by the rdtsc instruction, and is not
> + * paravirtualized.
> + */
> +#define CLOCK_PERF_HW_CLOCK		0x10000000
> +
>  /*
>   * The format of the data returned by read() on a perf event fd,
>   * as specified by attr.read_format:
> @@ -409,7 +418,8 @@ struct perf_event_attr {
>  				inherit_thread :  1, /* children only inherit if cloned with CLONE_THREAD */
>  				remove_on_exec :  1, /* event is removed from task on exec */
>  				sigtrap        :  1, /* send synchronous SIGTRAP on event */
> -				__reserved_1   : 26;
> +				ns_clockid     :  1, /* non-standard clockid */
> +				__reserved_1   : 25;
>  
>  	union {
>  		__u32		wakeup_events;	  /* wakeup every n events */

Thomas, do we want to gate this behind this magic flag, or can that
CLOCKID be granted unconditionally?