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

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