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

From: Vladimir Davydov
Date: Fri Jul 20 2012 - 14:22:06 EST

On Jul 20, 2012, at 9:20 PM, H. Peter Anvin wrote:

> 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.

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?

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.

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.

> -hpa

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