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

[Vladimir Davydov <vdavydov@parallels.com>]

On 07/21/2012 02:37 PM, Borislav Petkov wrote:
> (+ Andre who's been doing some cross vendor stuff)
>
> On Fri, Jul 20, 2012 at 08:37:33PM +0400, 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.
> As they say in Star Wars: "I have a bad feeling about this."
>
> So opening the floodgates to people fiddling with this (not only
> migrators) makes me feel pretty uneasy. And I won't wonder if all of
> a sudden strange failures start to appear because code is querying
> cpuid features but some funny distro has disabled it in its kernel boot
> options.
>
> Or some other obscure case where the culprit is hidden in kernel command
> line options.
>
> If it were only needed for migration, then I'd say you guys can use
> msr-tools and run a script as root on the target machine to which you
> want to migrate to and toggle the feature bits you want.

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. So, IMO, 
it would be better to have such functionality implemented in the kernel.

> I don't think cross vendor migration alone justifies having a generic
> kernel feature like that.
>
> Thanks.
>