linux-kernel - Re: [PATCH 2/2] cpu: intel, amd: mask cleared cpuid features

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Message-ID: <5009BD48.80204@zytor.com>
Date:	Fri, 20 Jul 2012 13:19:20 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Vladimir Davydov <VDavydov@...allels.com>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Borislav Petkov <borislav.petkov@....com>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] cpu: intel, amd: mask cleared cpuid features

On 07/20/2012 11:21 AM, Vladimir Davydov wrote:
>>
>> I am a bit concerned about this patch:
>>
>> 1. it silently changes existing behavior.
>
> Yes, but who needs the current implementation of 'clearcpuid' which,
> in fact, just hides flags in /proc/cpuinfo while userspace apps will
> see and consequently use all CPU features?

Anyone who wants to disable a feature from the kernel, specifically.

> So, I think it logically extends the existing behavior.
>
>> 2. even on enabled hardware, only some of the bits are maskable.
>
> The patch makes only words 0, 1, 4, 6 maskable, but words 3, 7, 8 are Linux-defined, words 2 and 5 are Transmeta-, Centaur-, etc- defined, and word 9 contains some bizarre Intel CPU features. Thus, it is words 0, 1, 4, 6 that contain useful information for most hardware models.

"Bizarre"?  New features, perhaps.

> If you ask about some Intel CPUs that can't mask CPUID function 0x80000001, this function describes AMD-specific features, and I bet those Intel CPUs just don't have them at all and thus have nothing to mask.

Not quite.

	-hpa



-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.



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