linux-kernel - Re: [PATCH v2 2/6] arm64: perf: Implement correct cap_user

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Date:   Wed, 15 Jul 2020 23:39:58 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul Cercueil <paul@...pouillou.net>,
        "Ben Dooks (Codethink)" <ben.dooks@...ethink.co.uk>,
        "Ahmed S. Darwish" <a.darwish@...utronix.de>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Kan Liang <kan.liang@...ux.intel.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/6] arm64: perf: Implement correct cap_user_time

Hi Peter,

On Wed, Jul 15, 2020 at 10:38:00AM +0200, Peter Zijlstra wrote:
> On Wed, Jul 15, 2020 at 10:05:08AM +0800, Leo Yan wrote:
> 
> > [leoyan: Use quot/rem to convert cyc to ns to avoid overflow]
> 
> > +		quot = rd->epoch_cyc >> rd->shift;
> > +		rem = rd->epoch_cyc & (((u64)1 << rd->shift) - 1);
> > +		ns = quot * rd->mult + ((rem * rd->mult) >> rd->shift);
> > +		userpg->time_zero -= ns;
> 
> I think we have mul_u64_u32_shr() for that.

Will fix it in next spin.

Thanks for suggestion,
Leo