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    SubjectRe: [RFC PATCH 18/33] Subarch support for CPUID instruction
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    From: Zachary Amsden <zach@vmware.com>
    Date: Tue, 18 Jul 2006 04:33:52 -0700

    > You really need a CPUID hook. The instruction is non-virtualizable, and
    > anything claiming to be a hypervisor really has to support masking and
    > flattening the cpuid namespace for the instruction itself. It is used
    > in assembler code and very early in boot. The alternative is injecting
    > a bunch of Xen-specific code to filter feature bits into the i386 layer,
    > which is both bad for Linux and bad for Xen - and was quite ugly in the
    > last set of Xen patches.

    Userspace will still see the full set of cpuid bits, since
    it can still execute cpuid unimpeded, is this ok?
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