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

From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Julien Grall <julien.grall@arm.com>, Jan Beulich <JBeulich@suse.com>
Cc: "Lars Kurth" <lars.kurth@citrix.com>,
	StefanoStabellini <sstabellini@kernel.org>,
	"Wei Liu" <wl@xen.org>,
	"George Dunlap" <George.Dunlap@eu.citrix.com>,
	Xen-devel <xen-devel@lists.xenproject.org>,
	"Ian Jackson" <ian.jackson@citrix.com>,
	"Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [Xen-devel] Terminology for "guest" - Was: [PATCH] docs/sphinx: Introduction
Date: Thu, 8 Aug 2019 11:30:59 +0100	[thread overview]
Message-ID: <aa9bb43f-bb0b-f5bc-0af4-7b54e26cda98@citrix.com> (raw)
In-Reply-To: <40816d88-b7e2-7d9f-2d7a-bede37a80e99@arm.com>

On 08/08/2019 10:13, 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.

Quite.  Even if you are using a hardware domain, dom0 is the control
domain.  Is this part of the host or not?

What about device driver domains?

To answer Jan's question, what is "fuzzy" is the fact that there is not
a clear definition.

> I actually thought we renamed everything in Xen from Dom0 to hwdom to
> avoid the 
>
> 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).

People thinking that dom0 isn't a VM is a massive problem for
understanding Xen's architecture, which is why I phrased some of
introduction the way I did.

It is a mistake which we need to take steps to address in our literature.

~Andrew

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