Message-ID: <49BA7810.6090807@goop.org>
Date: Fri, 13 Mar 2009 08:13:20 -0700
From: Jeremy Fitzhardinge <jeremy@goop.org>
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To: Jan Beulich <jbeulich@novell.com>
CC: "H. Peter Anvin" <hpa@zytor.com>,
       Xen-devel <xen-devel@lists.xensource.com>,
       Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>,
       the arch/x86 maintainers <x86@kernel.org>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [Xen-devel] [PATCH 10/24] xen: mask XSAVE from cpuid
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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.

    J
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