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

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Date:	Fri, 20 Jul 2012 10:20:33 -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,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] cpu: intel, amd: mask cleared cpuid features

On 07/20/2012 09:37 AM, Vladimir Davydov wrote:
> If 'clearcpuid=N' is specified in boot options, CPU feature #N won't be
> reported in /proc/cpuinfo and used by the kernel. However, if a
> userpsace process checks CPU features directly using the cpuid
> instruction, it will be reported about all features supported by the CPU
> irrespective of what features are cleared.
> 
> The patch makes the clearcpuid boot option not only clear CPU features
> in kernel but also mask them in hardware for Intel and AMD CPUs that
> support it so that the features cleared won't be reported even by the
> cpuid instruction.
> 
> This can be useful for migration of virtual machines managed by
> hypervisors that do not support/use Intel VT/AMD-V hardware-assisted
> virtualization technology.
> 
> If CPUID masking is supported, this will be reported in
> /proc/cpuinfo:flags as 'cpuidmask'.

I am a bit concerned about this patch:

1. it silently changes existing behavior.
2. even on enabled hardware, only some of the bits are maskable.

	-hpa


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