Re: [Xen-devel] Terminology for "guest" - Was: [PATCH] docs/sphinx: Introduction

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 8/8/19 10:13 AM, Julien Grall wrote:
> Hi Jan,
> 
> On 08/08/2019 10:04, Jan Beulich wrote:
>> On 08.08.2019 10:43, Andrew Cooper wrote:
>>> On 08/08/2019 07:22, Jan Beulich wrote:
>>>> On 07.08.2019 21:41, Andrew Cooper wrote:
>>>>> --- /dev/null
>>>>> +++ b/docs/glossary.rst
>>>>> @@ -0,0 +1,37 @@
>>>>> +Glossary
>>>>> +========
>>>>> +
>>>>> +.. Terms should appear in alphabetical order
>>>>> +
>>>>> +.. glossary::
>>>>> +
>>>>> +   control domain
>>>>> +     A :term:`domain`, commonly dom0, with the permission and
>>>>> responsibility
>>>>> +     to create and manage other domains on the system.
>>>>> +
>>>>> +   domain
>>>>> +     A domain is Xen's unit of resource ownership, and generally has
>>>>> at the
>>>>> +     minimum some RAM and virtual CPUs.
>>>>> +
>>>>> +     The terms :term:`domain` and :term:`guest` are commonly used
>>>>> +     interchangeably, but they mean subtly different things.
>>>>> +
>>>>> +     A guest is a single virtual machine.
>>>>> +
>>>>> +     Consider the case of live migration where, for a period of
>>>>> time, one
>>>>> +     guest will be comprised of two domains, while it is in transit.
>>>>> +
>>>>> +   domid
>>>>> +     The numeric identifier of a running :term:`domain`.  It is
>>>>> unique to a
>>>>> +     single instance of Xen, used as the identifier in various APIs,
>>>>> and is
>>>>> +     typically allocated sequentially from 0.
>>>>> +
>>>>> +   guest
>>>>> +     See :term:`domain`
>>>>
>>>> I think you want to mention the usual distinction here: Dom0 is,
>>>> while a domain, commonly not considered a guest.
>>>
>>> To be honest, I had totally forgotten about that.  I guess now is the
>>> proper time to rehash it in public.
>>>
>>> I don't think the way it currently gets used has a clear or coherent set
>>> of rules, because I can't think of any to describe how it does get used.
>>>
>>> Either there are a clear and coherent (and simple!) set of rules for
>>> what we mean by "guest", at which point they can live here in the
>>> glossary, or the fuzzy way it is current used should cease.
>>
>> What's fuzzy about Dom0 not being a guest (due to being a part of the
>> host instead)?
> Dom0 is not part of the host if you are using an hardware domain. I
> actually thought we renamed everything in Xen from Dom0 to hwdom to
> avoid the confusion.
> 
> I also would rather avoid to treat "dom0" as not a guest. In normal
> setup this is a more privilege guest, in other setup this may just be a
> normal guest (think about partitioning).

A literal guest is someone who doesn't live in the building (or work in
the buliding, if you're in a hotel).  The fact that the staff cleaning
rooms are restricted in their privileges doesn't make them guests of the
hotel.

The toolstack domain, the hardware domain, the driver domain, the
xenstore domain, and so on are all part of the host system, designed to
allow you to use Xen to do the thing you actually want to do: Run guests.

And it's important that we have a word that distinguishes "domains that
we only care about because they make it possible to run other domains",
and "domains that we actually want to run".  "guest / host" is a natural
terminology for these.

We already have the word "domain", which includes dom0, driver domains,
toolstack domains, hardware domains, as well as guest domains.  We don't
need "guest" to be a synonym for "domain".

 -George

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