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

To: Jan Beulich <jbeulich@xxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH 10/24] xen: mask XSAVE from cpuid
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Fri, 13 Mar 2009 08:13:20 -0700
Cc: the arch/x86 maintainers <x86@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>, Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, "H. Peter Anvin" <hpa@xxxxxxxxx>
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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.


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