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

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Message-ID: <YiIGwyhOrYid5qyF@hirez.programming.kicks-ass.net>
Date:   Fri, 4 Mar 2022 13:32:03 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Adrian Hunter <adrian.hunter@...el.com>
Cc:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        kvm@...r.kernel.org, H Peter Anvin <hpa@...or.com>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Leo Yan <leo.yan@...aro.org>
Subject: Re: [PATCH V2 02/11] perf/x86: Add support for TSC as a perf event
 clock

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?