linux-kernel - Re: [RFC 4/7] arm64: pmu: Add function implementation to update event index in userpage.

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Message-ID: <20190529094659.GK2623@hirez.programming.kicks-ass.net>
Date:   Wed, 29 May 2019 11:46:59 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Raphael Gault <raphael.gault@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        mingo@...hat.com, catalin.marinas@....com, will.deacon@....com,
        acme@...nel.org, mark.rutland@....com
Subject: Re: [RFC 4/7] arm64: pmu: Add function implementation to update
 event index in userpage.

On Tue, May 28, 2019 at 04:03:17PM +0100, Raphael Gault wrote:
> +static int armv8pmu_access_event_idx(struct perf_event *event)
> +{
> +	if (!(event->hw.flags & ARMPMU_EL0_RD_CNTR))
> +		return 0;
> +
> +	/*
> +	 * We remap the cycle counter index to 32 to
> +	 * match the offset applied to the rest of
> +	 * the counter indeces.
> +	 */
> +	if (event->hw.idx == ARMV8_IDX_CYCLE_COUNTER)
> +		return 32;
> +
> +	return event->hw.idx;

Is there a guarantee event->hw.idx is never 0? Or should you, just like
x86, use +1 here?

> +}