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

["H. Peter Anvin" <hpa@zytor.com>]

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