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

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Message-ID: <20090317115621.GQ11935@one.firstfloor.org>
Date:	Tue, 17 Mar 2009 12:56:21 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Jan Beulich <jbeulich@...ell.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH 10/24] xen: mask XSAVE from cpuid

> The point is YOU DON'T KNOW.  In particular, there might be new traps,
> there might be new state, there might be new MSRs, there might be new
> control bits... anything.  Therefore, you cannot blindly pass the bit
> on, even though XSAVE solves one part of the problem.

I think what will happen if you don't expose it is that there will
be always hypervisors which are behind and applications/OS will end up
doing probing for opcodes instead of trusting CPUID bits.

Probably not what you intended.

-Andi
-- 
ak@...ux.intel.com -- Speaking for myself only.
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