linux-kernel - Re: [PATCH v3 2/6] perf tsc: Add rdtsc() for Arm64

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Message-ID: <20200904190826.GC3753976@kernel.org>
Date:   Fri, 4 Sep 2020 16:08:26 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Leo Yan <leo.yan@...aro.org>
Cc:     peterz@...radead.org, Mark Rutland <mark.rutland@....com>,
        Ingo Molnar <mingo@...hat.com>, Wei Li <liwei391@...wei.com>,
        Al Grant <al.grant@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Kemeng Shi <shikemeng@...wei.com>,
        Ian Rogers <irogers@...gle.com>,
        John Garry <john.garry@...wei.com>,
        Stephane Eranian <eranian@...gle.com>,
        Nick Gasson <nick.gasson@....com>,
        Andi Kleen <ak@...ux.intel.com>,
        Steve MacLean <Steve.MacLean@...rosoft.com>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Will Deacon <will@...nel.org>,
        James Clark <james.clark@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/6] perf tsc: Add rdtsc() for Arm64

Em Thu, Sep 03, 2020 at 10:23:54AM +0800, Leo Yan escreveu:
> Hi Peter,
> 
> On Wed, Sep 02, 2020 at 03:48:05PM +0200, Peter Zijlstra wrote:
> > On Wed, Sep 02, 2020 at 02:21:27PM +0100, Leo Yan wrote:
> > > The system register CNTVCT_EL0 can be used to retrieve the counter from
> > > user space.  Add rdtsc() for Arm64.
> > 
> > > +u64 rdtsc(void)
> > > +{
> > > +	u64 val;
> > 
> > Would it make sense to put a comment in that this counter is/could-be
> > 'short' ? Because unlike x86-TSC, this thing isn't architecturally
> > specified to be 64bits wide.
> 
> Will add below comments:
> 
> According to ARM DDI 0487F.c, from Armv8.0 to Armv8.5 inclusive, the
> system counter is at least 56 bits wide; from Armv8.6, the counter must
> be 64 bits wide.  So the system counter could be less than 64 bits wide
> and it is attributed with the flag 'cap_user_time_short' is true.

Ok, so waiting for v4.

- Arnaldo