Re: [Xen-devel] [PATCH 10/24] xen: mask XSAVE from cpuid

Jeremy Fitzhardinge
2009-03-13T15:13:33Z

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.

J