Date: Tue, 17 Mar 2009 12:56:21 +0100
From: Andi Kleen <andi@firstfloor.org>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Andi Kleen <andi@firstfloor.org>, Jan Beulich <jbeulich@novell.com>,
       Arjan van de Ven <arjan@infradead.org>,
       Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>,
       Jeremy Fitzhardinge <jeremy@goop.org>,
       the arch/x86 maintainers <x86@kernel.org>,
       Xen-devel <xen-devel@lists.xensource.com>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [Xen-devel] [PATCH 10/24] xen: mask XSAVE from cpuid
Message-ID: <20090317115621.GQ11935@one.firstfloor.org>
References: <49BA3A84.76E4.0078.0@novell.com> <49BA7810.6090807@goop.org> <49BD4CE1.6040100@zytor.com> <49BD6D0E.1010107@goop.org> <20090315154718.00353625@infradead.org> <49BD97C6.2070401@goop.org> <20090315170945.08344b86@infradead.org> <49BE6D50.76E4.0078.0@novell.com> <87fxhd2f53.fsf@basil.nowhere.org> <49BEFDFB.8070900@zytor.com>
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> 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@linux.intel.com -- Speaking for myself only.
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