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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 13/01/16 17:03, Boris Ostrovsky wrote:
> On 01/13/2016 11:30 AM, Andrew Cooper wrote:
>> On 13/01/16 16:26, Jan Beulich wrote:
>>>>>> 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.
>> Thats perfectly fine and normal.  The same applies to all other domains
>> which wont migrate, or will only migrate to identical hardware.
>
> We don't use CPUID to only turn on and off features. It also, for
> example, describes topology -- something that we may use one day
> (possibly for dom0 too).

It still doesn't make dom0 special compared to other guests.  All guests
(should) have accurate topology information represented in cpuid (I will
also be fixing this).

~Andrew

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