Re: [Xen-devel] xenstore domain

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 09/12/15 07:34, Juergen Gross wrote:
> On 08/12/15 17:34, Andrew Cooper wrote:
>> On 08/12/15 16:02, Juergen Gross wrote:
>>> On 08/12/15 16:04, Andrew Cooper wrote:
>>>> On 08/12/15 14:44, Juergen Gross wrote:
>>>>> I'm just playing a little bit with xenstore in an own domain.
>>>>>
>>>>> I've come across some questions I'd like to have some answers to before
>>>>> presenting official patches to make this an easy configurable option:
>>>>>
>>>>> a) As this would need a boot time configuration item I'd like to add
>>>>>    e.g. /etc/xen/server.conf where such global configuration options
>>>>>    could be set via directives. Is this generally okay? If yes, which
>>>>>    format? Easiest way would be entries like
>>>>>    VAR=value
>>>>>    which can be either sourced in from shell scripts or can easily be
>>>>>    parsed in all programming languages. What are the preferences here?
>>>> Any configuration like this going to be toolstack-specific.  I would
>>>> recommend against using a name as generic as that.
>>>>
>>>> /etc/xl.conf already exists, which IMO would be the natural place for
>>>> this to live, but it isn't parseable by shell, because of vif notation.
>>> OTOH that file wouldn't be just for xl. It would be consumed by e.g.
>>> xencommons. Other configuration options I'd plan to add would be
>>> driver domains dedicated to specific interface cards.
>> It is still logically part of the "xl toolstack infrastructure", but I
>> accept your point.  The current xl.conf is all about how to create
>> domains in general, rather than specifically "how I would like my system
>> configured when starting up".
>>
>>>> One option might be to alter xl.conf to be compatible with shell
>>>> parsing.  It wouldn't be complicated (even in upgrade situations), and
>>>> would offer rather more flexibility.
>>> Shell parsing could be even handled via a rather simple filter, I guess.
>>>
>>>>> b) Today init-xenstore-domain will require flask to be enabled. An
>>>>>    alternative would be to add a new domain creation flag to allow the
>>>>>    domains with that flag set calling xc_domain_getinfo(). Thoughts?
>>>> Which flag?
>>> A new domcr_flag.
>> Indicating what, precisely?
> What I need is the capability to do the XEN_DOMCTL_getdomaininfo
> hypercall from the xenstore domain. Question is whether it's better
> to tie this special capability to the flag or to name it "is_xenstore".
>
> Thinking more about it, especially regarding a possible enhancement
> allowing Dom0 to reboot, I think the is_xenstore variant would be
> better. This would allow to look whether a xenstore domain is already
> running and connect to that rather than try to start a new one.

If we do indeed want dom0 to be able to reboot, then we definitely do
need some bit of remaining state indicating where xenstore is.

Currently it is the residual knowledge that dom0 has from whether it
started a local daemon, or a stubdomain, but that information would
disappear on a reboot.

In general, I would be against adding extra magic like this to Xen, but
xenstore is already sufficiently magic and critical in a Xen system that
the benefits of this special case probably do outweigh its downsides. 
(I wonder how long it will be until multiple xenstore domains are
suggested in earnest on xen-devel.)

~Andrew

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