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

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Date:	Sun, 15 Mar 2009 15:47:18 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Jan Beulich <jbeulich@...ell.com>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH 10/24] xen: mask XSAVE from cpuid

On Sun, 15 Mar 2009 14:03:10 -0700
Jeremy Fitzhardinge <jeremy@...p.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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