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

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Date:   Wed, 9 Feb 2022 14:11:13 +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 02/11] perf/x86: Add support for TSC as a perf event clock

On Wed, Feb 09, 2022 at 10:49:20AM +0200, Adrian Hunter wrote:
> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> index 82858b697c05..150d2b70a41f 100644
> --- a/include/uapi/linux/perf_event.h
> +++ b/include/uapi/linux/perf_event.h
> @@ -290,6 +290,14 @@ 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.
> + * On x86, this is provided by the rdtsc instruction, and is not
> + * paravirtualized.
> + */
> +#define CLOCK_PERF_HW_CLOCK		0x10000000

This steps on the clockid_t space; are we good with that?

At some point there was talk of dynamic clock ids, that would complicate
things more than they are today.