linux-kernel - Re: [PATCH] perf/x86: fix microcode revision check for SNB-PEBS

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Date:	Fri, 24 Aug 2012 18:14:51 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Borislav Petkov <bp@...64.org>
Cc:	linux-kernel@...r.kernel.org, peterz@...radead.org, mingo@...e.hu,
	andi@...stfloor.org
Subject: Re: [PATCH] perf/x86: fix microcode revision check for SNB-PEBS

On Fri, Aug 24, 2012 at 6:08 PM, Borislav Petkov <bp@...64.org> wrote:
> On Fri, Aug 24, 2012 at 03:34:34PM +0200, Stephane Eranian wrote:
>>
>> The following patch, relative to 3.6.0-rc3, makes
>> the microcode update code path actually invoke the
>> perf_check_microcode() function and thus potentially
>> renabling SNB PEBS.
>>
>> By default, CONFIG_MICROCODE_OLD_INTERFACE is
>> forced to Y in arch/x86/Kconfig. There is no
>> way to disable this. That means that the code
>> path used in arch/x86/kernel/microcode_core.c
>> did not include the call to perf_check_microcode().
>>
>> Thus, even though the microcode was updated to a
>> version that fixes the SNB PEBS problem, perf_event
>> would still return EOPNOTSUPP when enabling precise
>> sampling.
>>
>> This patch simply adds a call to perf_check_microcode()
>> in the call path used when OLD_INTERFACE=y.
>
> Ok, so c93dc84cbe324 added calls to perf_check_microcode but it looks
> like you're updating the microcode from /dev/cpu/microcode, correct?
>
> And if so, the old interface got missed.
>

So you're saying there is a cmdline tool that can use another interface
to pass the ucode to the kernel. I am using the microcode_ctl on Ubuntu
11.04. Apparently that one is still using the old interface. But I may
not be the
only one in this situation then, so I think it's worth fixing now.

What's the tool to update the ucode using the "new" interface then?

Thanks.

> Oh well, as long as we have to support it, we might as well add that
> perf call there - it will go when the interface goes anyway so until
> then:
>
> Acked-by: Borislav Petkov <borislav.petkov@....com>
>
>>
>> Signed-off-by: Stephane Eranian <eranian@...gle.com>
>> ---
>>
>> --- a/arch/x86/kernel/microcode_core.c
>> +++ b/arch/x86/kernel/microcode_core.c
>> @@ -225,6 +225,9 @@ static ssize_t microcode_write(struct file *file, const char __user *buf,
>>       if (do_microcode_update(buf, len) == 0)
>>               ret = (ssize_t)len;
>>
>> +     if (ret > 0)
>> +             perf_check_microcode();
>> +
>>       mutex_unlock(&microcode_mutex);
>>       put_online_cpus();
>>
>>
>
> Thanks.
>
> --
> Regards/Gruss,
> Boris.
>
> Advanced Micro Devices GmbH
> Einsteinring 24, 85609 Dornach
> GM: Alberto Bozzo
> Reg: Dornach, Landkreis Muenchen
> HRB Nr. 43632 WEEE Registernr: 129 19551
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