Re: [PATCH] perf/x86: check ucode before disabling PEBS onSandyBridge

[Robert Richter]

On 08.06.12 15:26:12, Peter Zijlstra wrote:
> +static const u32 snb_ucode_rev = 0x28;
> +
> +static void intel_snb_verify_ucode(void)
> +{
> +	u32 rev = UINT_MAX;
> +	int pebs_broken = 0;
> +	int cpu;
> +
> +	get_online_cpus();
> +	/*
> +	 * Because the microcode loader is bloody stupid and allows different
> +	 * revisions per cpu and does strictly per-cpu loading, we now have to
> +	 * check all cpus to determine the minimally installed revision.
> +	 *
> +	 * This makes updating the microcode O(n^2) in the number of CPUs :/
> +	 */
> +	for_each_online_cpu(cpu)
> +		rev = min(cpu_data(cpu).microcode, rev);
> +	put_online_cpus();
> +
> +	pebs_broken = (rev < snb_ucode_rev);
> +
> +	if (pebs_broken == x86_pmu.pebs_broken)
> +		return;
> +
> +	/*
> +	 * Serialized by the microcode lock..
> +	 */
> +	if (x86_pmu.pebs_broken) {
> +		pr_info("PEBS enabled due to micro-code update\n");
> +		x86_pmu.pebs_broken = 0;
> +	} else {
> +		pr_info("PEBS disabled due to CPU errata, "
> +			"please upgrade micro-code to at least %x (current: %x)\n",
> +			snb_ucode_rev, rev);
> +		x86_pmu.pebs_broken = 1;
> +	}
> +}
> +
> +static int intel_snb_ucode_notifier(struct notifier_block *self,
> +				   unsigned long action, void *_uci)
> +{
> +	/*
> +	 * Since ucode cannot be down-graded, and no future ucode revision
> +	 * is known to break PEBS again, we're ok with MICROCODE_CAN_UPDATE.
> +	 */
> +
> +	if (action == MICROCODE_UPDATED)
> +		intel_snb_verify_ucode();
> +
> +	return NOTIFY_DONE;
> +}
> +
>  static __init void intel_sandybridge_quirk(void)
>  {
> -	pr_warn("PEBS disabled due to CPU errata\n");
> -	x86_pmu.pebs = 0;
> -	x86_pmu.pebs_constraints = NULL;
> +	intel_snb_verify_ucode();
> +	/*
> +	 * we're still single threaded, so while there's a hole here,
> +	 * you can't trigger it.
> +	 */
> +	microcode_notifier(intel_snb_ucode_notifier);
>  }

Instead of registering a microcode notifier, why not checking the
availability of pebs dynamically with each syscall in
intel_pmu_hw_config()? It looks like intel_snb_verify_ucode() is not
that much expensive. We can perform the check only if the event could
be for pebs and if pebs is broken. The check could be repeated when
setting up a new event after ucode could potentially has been updated
(e.g. after bringing a cpu online or so).

-Robert


-- 
Advanced Micro Devices, Inc.
Operating System Research Center

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