linux-kernel - Re: [PATCH] perf/x86: check ucode before disabling PEBS on SandyBridge

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Date:	Tue, 12 Jun 2012 19:09:19 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Robert Richter <robert.richter@....com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	andi@...stfloor.org, mingo@...e.hu, ming.m.lin@...el.com,
	Andreas Herrmann <andreas.herrmann3@....com>,
	Borislav Petkov <borislav.petkov@....com>,
	Dimitri Sivanich <sivanich@....com>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>
Subject: Re: [PATCH] perf/x86: check ucode before disabling PEBS on SandyBridge

On Tue, Jun 12, 2012 at 7:07 PM, Robert Richter <robert.richter@....com> wrote:
> 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).
>
That's what I had in my original version.

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