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

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Message-ID: <49BA7810.6090807@goop.org>
Date:	Fri, 13 Mar 2009 08:13:20 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Jan Beulich <jbeulich@...ell.com>
CC:	"H. Peter Anvin" <hpa@...or.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

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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