linux-kernel - Re: [PATCH V6 2/5] perf/x86/kvm: Avoid unnecessary work in guest filtering

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Date:   Mon, 4 Feb 2019 11:55:27 -0500
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, tglx@...utronix.de,
        bp@...en8.de, mingo@...hat.com, ak@...ux.intel.com,
        eranian@...gle.com
Subject: Re: [PATCH V6 2/5] perf/x86/kvm: Avoid unnecessary work in guest
 filtering



On 2/4/2019 11:23 AM, Peter Zijlstra wrote:
> On Mon, Feb 04, 2019 at 10:57:32AM -0500, Liang, Kan wrote:
>>
>>
>> On 2/4/2019 10:44 AM, Peter Zijlstra wrote:
>>> On Mon, Feb 04, 2019 at 04:38:27PM +0100, Peter Zijlstra wrote:
>>>> +static const struct x86_cpu_desc isolation_ucodes[] = {
>>>> +	INTEL_CPU_DESC(INTEL_FAM6_KABYLAKE_MOBILE,	 9, 0x0000004e),
>>>> +	INTEL_CPU_DESC(INTEL_FAM6_KABYLAKE_MOBILE,	10, 0x0000004e),
>>>> +	INTEL_CPU_DESC(INTEL_FAM6_KABYLAKE_MOBILE,	11, 0x0000004e),
>>>> +	INTEL_CPU_DESC(INTEL_FAM6_KABYLAKE_MOBILE,	12, 0x0000004e),
>>>
>>>> +	INTEL_CPU_DESC(INTEL_FAM6_KABYLAKE_DESKTOP,	10, 0x0000004e),
>>>> +	INTEL_CPU_DESC(INTEL_FAM6_KABYLAKE_DESKTOP,	11, 0x0000004e),
>>>> +	INTEL_CPU_DESC(INTEL_FAM6_KABYLAKE_DESKTOP,	12, 0x0000004e),
>>>> +	INTEL_CPU_DESC(INTEL_FAM6_KABYLAKE_DESKTOP,	13, 0x0000004e),
>>>
>>> Do we want a special stepping (0 / -1) to be able to denote 'all' ?
>>>
>>
>> Something like as below?
>> #define X86_STEPPING_ANY	0xff
>>
>> As my understanding, the microcode version for each stepping is independent
>> and irrelevant. The 0x0000004e should be just coincidence.
>> If so, I don't think X86_STEPPING_ANY is very useful.
> 
> Sure; but since we have this happy accident, we can still use it for a
> notational convenience, right?

We cannot apply X86_STEPPING_ANY to ignore the stepping. There will be 
problems for 0-8 stepping for KABYLAKE_MOBILE.
I think what we need is x86_match_cpu_with_stepping_range().
But I don't think it is worth enabling it just for this rare case.

Thanks,
Kan