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[Xen-devel] Re: One question on xen_cpuid in pv guest

To: "Jiang, Yunhong" <yunhong.jiang@xxxxxxxxx>
Subject: [Xen-devel] Re: One question on xen_cpuid in pv guest
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Wed, 29 Jul 2009 09:54:55 -0700
Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
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On 07/28/09 20:19, Jiang, Yunhong wrote:
> I have a question to xen_cpuid() in pv_ops guest. Currently the cpuid will be 
> trapped to xen HV for emulation. So why do we still use 
> cpuid_leaf1_edx_mask/cpuid_leaf1_ecx_mask to do mask in guest side? Havn't 
> that been achieved in hypervisor already?

Not entirely, or consistently.  In general Xen masks features which are
outright forbidden for the guest to use, but leaves things that might
possibly be useful - we still need to mask those out.  It also fails to
mask some newer features which are not usable in guests (like GByte
pages and XSAVE), so we need to mask those anyway.  For domU, the set of
mask flags is under the control of the domain config file; a
misconfiguration could enable flags that the kernel can simply not use.


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