linux-kernel - Re: [PATCH v2 0/7] perf arm-spe: Enable timestamp

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Message-ID: <20210407131529.GA585163@leoy-ThinkPad-X240s>
Date:   Wed, 7 Apr 2021 21:15:29 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     Al Grant <Al.Grant@....com>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        John Garry <john.garry@...wei.com>,
        Will Deacon <will@...nel.org>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        James Clark <James.Clark@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <Mark.Rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 0/7] perf arm-spe: Enable timestamp

Hi Al,

On Tue, Apr 06, 2021 at 09:38:32AM +0000, Al Grant wrote:

[...]

> > This patch set is to enable timestamp for Arm SPE trace.  It reads out TSC
> > parameters from mmap page and stores into auxtrace info structure;
> 
> Why not synthesize a PERF_RECORD_TIME_CONV - isn't that specifically to
> capture the TSC parameters from the mmap page? If a generic mechanism
> exists it would be better to use it, otherwise we'll have to do this again for
> future trace formats.

Good point!  Actually "perf record" tool has synthesized event
PERF_RECORD_TIME_CONV.  This patch series is studying the
implementation from Intel-PT, so the question is why the existed
implementations (like Intel-PT, Intel-BTS) don't directly use
PERF_RECORD_TIME_CONV for retriving TSC parameters.

I agree using PERF_RECORD_TIME_CONV for TSC parameter is better than
extending auxtrace info.  Will experiment for this.

> perf_read_tsc_conversion and perf_event__synth_time_conv are currently
> in arch/x86/util/tsc.c, but nothing in them is x86-specific and they could be
> moved somewhere more generic.

This is not true on the mainline kernel; these functions have been
moved into the file util/tsc.c.

Thanks for suggestions,
Leo