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

To: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Subject: [Xen-devel] RE: One question on xen_cpuid in pv guest
From: "Jiang, Yunhong" <yunhong.jiang@xxxxxxxxx>
Date: Thu, 30 Jul 2009 13:44:07 +0800
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Thread-topic: One question on xen_cpuid in pv guest
Jeremy, thanks for clarification, but I still not sure about following code in 
pv_ops dom0. Since the Local APIC is owned by Xen totally, why is it not 
"outright forbidden for the guest"?

    if (!xen_initial_domain())
        cpuid_leaf1_edx_mask &=
            ~((1 << X86_FEATURE_APIC) |  /* disable local APIC */
              (1 << X86_FEATURE_ACPI));  /* disable ACPI */

Yunhong Jiang

Jeremy Fitzhardinge wrote:
> 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. 
>    J
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