startupPolicy issue when changing CD

[Peter Krempa]

On Fri, Sep 10, 2021 at 15:34:57 +0200, Vojtech Juranek wrote:
> On Friday, 10 September 2021 15:12:07 CEST Peter Krempa wrote:
> > On Fri, Sep 10, 2021 at 14:53:23 +0200, Vojtech Juranek wrote:
> > > Hi,
> > > when adding support for CD disk on block based storage into oVirt,
> > > I spotted following issue. When starting VM without CD, we add
> > > startupPolicy='optional' attribute into <source> element.
> > > 
> > > Whole XML looks like this:
> > >     <disk type='file' device='cdrom'>
> > >     
> > >       <driver name='qemu' error_policy='report'/>
> > >       <source startupPolicy='optional'/>
> > >       <target dev='sdc' bus='sata'/>
> > >       <readonly/>
> > >       <alias name='ua-d7003457-63c3-4c98-80f0-08ab5042c65e'/>
> > >       <address type='drive' controller='0' bus='0' target='0' unit='2'/>
> > >     
> > >     </disk>
> > > 
> > > To change/insert CD we use libvirt.updateDeviceFlags() with XML which
> > > looks like this (for block based disk):
> > > 
> > > <?xml version='1.0' encoding='utf-8'?>
> > > <disk device="cdrom" type="block">
> > > 
> > >     <source
> > >     dev="/rhev/data-center/mnt/blockSD/cdac2a0c-b110-456d-a988-7d588626c8
> > >     71/images/638247d7-b4b1-4d98-87fa-c90235fcf4b1/145e7cd2-f92d-4eec-a8fb
> > >     -6835b4b652e1" /> <target bus="sata" dev="sdc" />

Please note that according to the updateDevice API docs you should
provide a fully-defined XML of the device, and not omit fields you don't
want to change.

This fact was even discussed today on the mailing list:

https://listman.redhat.com/archives/libvir-list/2021-September/msg00246.html

> > > 
> > > </disk>
> > > 
> > > However, updateDeviceFlags() fails with
> > > 
> > >     libvirt.libvirtError: XML error: 'startupPolicy' is only valid for
> > >     'file' type volume> 
> > > What is the reason for this error? We don't use `startupPolicy` attribute
> > > for block based disks, as shown on example above.
> > 
> > Hmm, the bug is that the disk source change is attempted before the
> > update to startup policy, thus the validator is unhappy.
> 
> is this a libvirt bug or intentional behavior and the CD change should be done 
> in two steps:
> * remove startupPolicy from the disk
> * do actual change of the disk
> ?

Fix proposed:

https://listman.redhat.com/archives/libvir-list/2021-September/msg00283.html