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

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Date:	Mon, 31 Aug 2015 22:04:15 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	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

On Mon, Aug 31, 2015 at 03:59:08PM +0300, Alexander Shishkin wrote:
> 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.

What I meant is -- the kernel may crash on a older CPU that does
not support CPUID 0x15. For example Broadwell doesn't have it.
For CPUID you always need to check the level.

-Andi
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