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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...


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

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