Re: [Xen-devel] A couple of HVMlite loose ends

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 13.01.16 at 17:17, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 13/01/16 16:13, Jan Beulich wrote:
>>>>> On 13.01.16 at 16:49, <roger.pau@xxxxxxxxxx> wrote:
>>> Hello,
>>>
>>> While working on a HVMlite Dom0 implementation I've found a couple of
>>> loose ends with the design that I would like to comment because it's not
>>> clear to me what's the best direction to take.
>>>
>>> 1. HVM CPUID and Dom0.
>>>
>>> Sadly the way CPUID is handled inside of Xen varies between PV and HVM.
>>> On PV guests AFAICT we mostly do black-listing (I think this is the
>>> right term), which means we pick the native CPUID result and then
>>> perform a series of filter operations in order to remove features which
>>> should not be exposed to a PV guest. On the other hand, for HVM guests
>>> we pre-populate an array (d->arch.cpuids) during domain build time, and
>>> the contents of that array is what is returned to the guest when a CPUID
>>> instruction is executed.
>> This d->arch.cpuids[] mechanism is common to HVM and PV; the
>> exception really is Dom0.
> 
> Dom0 is not special when it comes to cpuid, and shouldn't be treated as
> such.  My longter term CPUID plans will be fixing this.

In some way it is - there's no need for hiding features from it, since
it can't be migrated.

Jan


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