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

Arjan van de Ven
2009-03-15T22:53:19Z

On Sun, 15 Mar 2009 14:03:10 -0700
Jeremy Fitzhardinge <jeremy@goop.org> wrote:

> H. Peter Anvin wrote:
> > 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...
> >
>
> Well, at the moment the problem is that cpuid (both PV and native)
> show XSAVE, but Xen prevents cr4.OSXSAVE from being set, crashing the
> kernel. There's now a patch in Xen to mask XSAVE in CPUID, so that
> guests don't try to use it; the patch in the kernel is just to
> support non-bleeding-edge versions of Xen.

This is indicative of something that might be a huge bug in Xen:
Xen should never ever pass through CPUID bits it does not know.
If Xen does not honor that, there is a fundamental and eternally
recurring problem.... every time something new gets introduced Xen
likely breaks.

--
Arjan van de Ven Intel Open Source Technology Centre
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