linux-kernel - Re: [PATCH 1/4] arm64: add support for the AMU extension v1

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Date:   Thu, 10 Oct 2019 18:20:58 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Ionela Voinescu <ionela.voinescu@....com>
Cc:     will@...nel.org, maz@...nel.org, corbet@....net,
        linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH 1/4] arm64: add support for the AMU extension v1

Hi Ionela,

On Tue, Sep 17, 2019 at 02:42:25PM +0100, Ionela Voinescu wrote:
> +#ifdef CONFIG_ARM64_AMU_EXTN
> +
> +/*
> + * This per cpu variable only signals that the CPU implementation supports the
> + * AMU but does not provide information regarding all the events that it
> + * supports.
> + * When this amu_feat per CPU variable is true, the user of this feature can
> + * only rely on the presence of the 4 fixed counters. But this does not
> + * guarantee that the counters are enabled or access to these counters is
> + * provided by code executed at higher exception levels.
> + */
> +DEFINE_PER_CPU(bool, amu_feat) = false;
> +
> +static void cpu_amu_enable(struct arm64_cpu_capabilities const *cap)
> +{
> +	if (has_cpuid_feature(cap, SCOPE_LOCAL_CPU)) {
> +		pr_info("detected CPU%d: Activity Monitors extension\n",
> +			smp_processor_id());
> +		this_cpu_write(amu_feat, true);
> +	}
> +}

Sorry if I missed anything as I just skimmed through this series. I
can't see the amu_feat used anywhere in these patches, so on its own it
doesn't make much sense.

I also can't see the advantage of allowing mismatched CPU
implementations for this feature. What's the intended use-case?

Thanks.

-- 
Catalin