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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: Thu, 30 Jul 2009 10:06:36 -0700
Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
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On 07/29/09 22:44, Jiang, Yunhong wrote:
> 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 */

Some of the approaches to implementing dom0 pretend the local APIC
exists in order to fool various code-paths into doing the right thing
(for example, getting the kernel to parse all the MADT tables, which it
won't do if there's no local APIC).

Historically I think the policy has been for Xen to explicitly blacklist
features which turn out to be problematic when they become problematic,
rather than whitelist features known to be safe.


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