linux-kernel - Re: [PATCH] perf/x86/intel/pt: Export cpu frequency ratios needed by PT decoders

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Message-ID: <87io7vhaw3.fsf@ashishki-desk.ger.corp.intel.com>
Date:	Mon, 31 Aug 2015 15:59:08 +0300
From:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Arnaldo Carvalho de Melo <acme@...radead.org>
Subject: Re: [PATCH] perf/x86/intel/pt: Export cpu frequency ratios needed by PT decoders

Andi Kleen <andi@...stfloor.org> writes:

> Alexander Shishkin <alexander.shishkin@...ux.intel.com> writes:
>> +		return sprintf(page, "%lu\n", val);
>> +	case 1:
>> +		cpuid(0x15, &eax, &ebx, &ecx, &edx);
>
> Surely this needs to be protected by a cpuid level check?
> Broadwell for once doesn't have CPUID 0x15.

I couldn't find where this is explicitly said in the SDM. However, the
leaf description says that CPUID.0x15.EBX[31:0]==0 means that the ratio
is not enumerated. I don't see how to better communicate this to
userspace though than to simply have 0 there.

Regards,
--
Alex
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