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Re: [Xen-devel] xenstore -- read UUID

To: Derek.Murray@xxxxxxxxxxxx
Subject: Re: [Xen-devel] xenstore -- read UUID
From: "Geoffrey Lefebvre" <geoffrey@xxxxxxxxx>
Date: Wed, 10 Sep 2008 08:31:11 -0700
Cc: xen developers community <xen-devel@xxxxxxxxxxxxxxxxxxx>, xen users community <xen-users@xxxxxxxxxxxxxxxxxxx>, dinesh chandrasekaran <dinesh_chan8@xxxxxxxxxxx>
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Hi everyone,

Sorry for the late joining to this thread.

If you really want to gather a domain's uuid from within the
hypervisor, it's actually possible to do without xenstore.
A domain's uuid gets stored as a xen_domain_handle_t in the domain
handle field (see xen/include/xen/sched.h and xen/commom/domctl.c).
The uuid of a domain gets passed to xen on domain creation. The uuid
can also be read from a privileged domain by issuing the
XEN_DOMCTL_getdomaininfo hypercall.

But as Derek mentioned earlier, you can read the domain uuid from a
guest via xenstore which is probably a better idea.



On Wed, Sep 10, 2008 at 4:49 AM, Derek Murray <Derek.Murray@xxxxxxxxxxxx> wrote:
> Hi Dinesh,
> On Wed, Sep 10, 2008 at 1:24 AM, dinesh chandrasekaran
> <dinesh_chan8@xxxxxxxxxxx> wrote:
>> they were 2 seperate questions... sorry for the typos...
>> How to access xenstore from Dom-0? -- which you said 'YES'...
> Yes, this is certainly possible: your code should work (I haven't
> tested it) in Dom0 user-space. For another example, see
> tools/xenstore/xsls.c, which implements ls functionality for xenstore
> using the same interface.
>> And How to access xenstore from hypervosor?
> There is no code to do this in Xen at the moment.
>> If the hypervisor has no knowledge about XenStore, Dom-0 must have the full
>> control over the xenstore.
> Correct. XenStore is simply implemented as a user-space daemon
> (xenstored), running as root in Dom0.
>> I need to build a table with some fields one of which should never change
>> during a VMs lifetime.
>> For example, if I chose the domain_id whose value is transient, If the VM
>> reboots, its domain_id changes.
>> That is the reason why I opted for the UUID of a domain that persists over
>> time even after migration to another machine.
>> The only way I could get a domain's UUID is through the xenstore and want to
>> take away domain-0 from the picture, which
>> according to you is almost impossible.
> Instead of Dom0, you could possibly do this in a stubdomain. It is
> possible to access XenStore from a MiniOS-based domain.
> Regards,
> Derek Murray.
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