Re: Domain XML of a restored VM
On 6/10/22 15:31, Tomáš Golembiovský wrote:
Hi,
I am looking for clarification on what exactly is happening to the
domain XML when resuming a suspended VM. We would like to make sure we
don't lose any configuration and we also see some unexpected behavior.
The VM with the name/UUID same as resumed VM may or may not already
exist. If it does, what role does the original domain XML of a VM play
when resuming? Is there any merging taking place between the existing
domain XML and the domain XML stored in save image? Is the original XML
simply thrown away and replaced?
A guest can have up to two XMLs:
1) defined guests have so called 'inactive' or 'config' XML. This XML
does not contain any runtime information, and is used as the initial
configuration when starting the guest. Once the guest is running, it'll
also have ...
2) live XML. This XML contains runtime information and because of
hotplug and plenty of APIs that tweak various XML knobs
(virDomainSetMemoryParameters(), virDomainSetInterfaceParameters(),
etc.) can be very different to the inactive XML. Changes to the live XML
are thrown away as soon as the guest is shut off and we're back to
square 1 where only the inactive XML is preserved (and used as the
initial config for the next run).
However, this live XML is very important because it reflects the guest
ABI. For instance, some of our APIs offer a way for users to supply
modified domain XML (as you say below, virDomainRestoreFlags() is onw of
them, migration APIs are another). After this user supplied XML is
parsed, it's thrown at an excessive ABI stability checker, who's only
goal is to make sure that the live XML maintained by Libvirt and the
user provided XML differ only in those ways that won't break guest ABI
(for instance, changing the port where SPICE is allowed, removing a
<disk/> isn't).
Having said all of this - the XML that's saved among the domain by
virDomainSave() API is the live XML. Because that's the one that
reflects the current state of the saved image. Inactive XML is
disconnected from all of this.
Is there any difference how the stored XML is treated compared to
the
XML passed in `virDomainRestoreFlags()`? I see some unexpected changes
here. When I use `virDomainSaveImageGetXMLDesc()` to retrieve the stored
XML and pass it unchanged to the `virDomainRestoreFlags()` the final
domain XML is different then when `virDomainRestore()` is used or when
`virDomainRestoreFlags()` with no `dxml` argument is used. We have:
<graphics ... passwdValidTo='1970-01-01T00:00:01'>
in the domain XML and the `passwdValidTo` argument disappears when
domain XML is passed in `dxml` argument but it is preserved in other
cases. Is this behavior expected? If yes, can we do anything to preserve
the configuration? What other changes in domain XML can be expected?
No, this behaviour is definitely not expected.
As explained above, runtime values, that don't affect guest ABI are
allowed to change. The rest is not. Password timeout should be allowed
to change, because guest has no idea about the password in the first
place. This is all done in the backed (that part of SPICE that faces the
host/clients).
Michal