Re: [Xen-devel] [PATCH 8/9] xen/arch/x86: use is_hardware_domain instead of domid == 0

[Keir Fraser <keir.xen@xxxxxxxxx>]

On 12/04/2013 15:49, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:

>> My impression was that this would be the hardware domain since it is
>> the one that would be parsing ACPI tables and similar things that need
>> CPUID to work. The control domain would just be involved in creating
>> and managing domains, which doesn't need anything out of CPUID that a
>> normal PV domain would need.
> 
> Parsing ACPI tables is unrelated to CPUID, as is most other
> hardware management. The only reason I could see to override
> this would be to have some domains know the true capabilities of
> the processor. And that would be the control domain(s), in order
> to be able to sensibly set up CPUID emulation for the guests.

Yes, it's subtle but this would break tools/libxc/xc_cpuid_x86.c which
expects to see the real CPUID, not the virtualised one. For example, it will
look for host 64-bit capabilities this way, which would be hidden in the
virtualised CPUID space if the control domain is 32-bit.

We could get rid of this not-always-faulting-cpuid hack in the hypervisor
altogether, if we could provide a paravirtualised interface that guarantees
access to the real CPUID space, for the toolstack to use.

But anyhow, the hack is for the toolstack only, and hence is for
control_domain only.

 -- Keir



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