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    SubjectRe: [Xen-devel] [PATCH 10/24] xen: mask XSAVE from cpuid
    Jeremy Fitzhardinge wrote:
    > Jan Beulich wrote:
    >> As pointed out on an earlier thread, it seems inappropriate to do probing
    >> like this when there is a cpuid feature flag (osxsave) that can be
    >> used to
    >> determine whether XSAVE can be used. And even without that flag,
    >> simply reading CR4 and checking whether osxsave is set there would
    >> suffice. This is under the assumption that Xen's to-be-done
    >> implementation
    >> of XSAVE support would match that of FXSAVE (Xen turns its support on
    >> unconditionally and for all [pv] guests).
    >
    > I didn't want to make too many assumptions about how Xen's XSAVE support
    > would look. In particular, I thought it might virtualize the state of
    > OSXSAVE to give the guest the honour of appearing to enable it. A guest
    > kernel may get confused if it starts with OSXSAVE set, as it may use it
    > to control its own init logic.

    That wouldn't be an issue if you use the *native* CPUID to look for
    OSXSAVE early on, since such virtualization would only be visible though
    the PV interface, right?

    It seems cleaner than probing, to be sure...

    -hpa

    --
    H. Peter Anvin, Intel Open Source Technology Center
    I work for Intel. I don't speak on their behalf.



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