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

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Date:	Tue, 24 Jul 2012 09:48:12 +0200
From:	Borislav Petkov <bp@...64.org>
To:	Vladimir Davydov <vdavydov@...allels.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.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>,
	André Przywara <andre.przywara@....com>,
	Andreas Herrmann <andreas.herrmann3@....com>
Subject: Re: [PATCH 2/2] cpu: intel, amd: mask cleared cpuid features

On Tue, Jul 24, 2012 at 11:06:08AM +0400, Vladimir Davydov wrote:
> If msr-tools are used for cpuid masking, we will either get
> inconsistency between /proc/cpuinfo:flags and the output of the cpuid
> instruction or have to "synchronize" the clearcpuid boot option and
> the userspace app using msr-tools, which seems to be inconvenient.

Right, so IMO what you want to do is not very kosher as a generic kernel
feature for reasons described earlier.

And yes, "synchronizing" /proc/cpuinfo's flags and CPUID output is a lot
less pain compared to having a generic abusable feature.

And also much better suited for your niche use case than opening this up
as a generic functionality to the wide public which will most definitely
find productive ways to abuse this.

Simply ask yourself this: is this something which the majority of
x86 linux users will benefit from or is it only something which is
nice-to-have solely for our single use case?

Thanks.

-- 
Regards/Gruss,
Boris.

Advanced Micro Devices GmbH
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