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Date:   Wed, 2 Sep 2020 15:48:05 +0200
From:   peterz@infradead.org
To:     Leo Yan <leo.yan@linaro.org>
Cc:     Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Ingo Molnar <mingo@redhat.com>, Wei Li <liwei391@huawei.com>,
        Al Grant <al.grant@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Adrian Hunter <adrian.hunter@intel.com>,
        "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Kemeng Shi <shikemeng@huawei.com>,
        Ian Rogers <irogers@google.com>,
        John Garry <john.garry@huawei.com>,
        Stephane Eranian <eranian@google.com>,
        Nick Gasson <nick.gasson@arm.com>,
        Andi Kleen <ak@linux.intel.com>,
        Steve MacLean <Steve.MacLean@Microsoft.com>,
        "Gustavo A. R. Silva" <gustavoars@kernel.org>,
        Kan Liang <kan.liang@linux.intel.com>,
        Will Deacon <will@kernel.org>,
        James Clark <james.clark@arm.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 2/6] perf tsc: Add rdtsc() for Arm64
Message-ID: <20200902134805.GI1362448@hirez.programming.kicks-ass.net>
References: <20200902132131.36304-1-leo.yan@linaro.org>
 <20200902132131.36304-3-leo.yan@linaro.org>
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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.

> +	asm volatile("mrs %0, cntvct_el0" : "=r" (val));
> +
> +	return val;
> +}
> -- 
> 2.17.1
>