[libvirt] [BUG] Fix order of disks and controllers

[Daniel P. Berrange]

On Mon, Oct 31, 2011 at 10:52:33AM +0100, Philipp Hahn wrote:
> Hello,
> 
> On Wednesday 01 June 2011 17:58:32 Jiri Denemark wrote:
> > Commit 2d6adabd53c8f1858191d521dc9b4948d1205955 replaced qsorting disk
> > and controller devices with inserting them at the right position. That
> > was to fix unnecessary reordering of devices. However, when parsing
> > domain XML devices are just taken in the order in which they appear in
> > the XML since. Use the correct insertion algorithm to honor device
> > target.
> 
> This commit (c1a98d88255197a8446d08c0b1589861660e9064) broke the Xen-PC-PyGrub 
> case (see also "[PATCH 3/4] xen: fix PyGrub boot device order" from Oct 12th 
> 2011, which worked only for preious versions of libvirt):
> XenD used the order of disks on domain-to-native to set the bootable flag to 1 
> only for the first device. This flag is then used on domain-start to 
> determine the disk, which gets passed ty PyGrub to extract the Kernel and 
> IniRD.
> Now the order is determined by /domain/devices/disk/target/@dev, which makes 
> chaning the order impossible without also changing the devices names, which 
> are visible to the VM! The Nr. 1 use-case for this is chaning the boot-order 
> after installing from a PV-CDROM to the PV-disk.
> Since the Xen-PV-case has no BIOS bootable section, using /domain/os/boot/ 
> here does not work.
> 
> Either that libvirt needs to explicitly support Xens bootable flag 
> (via /domain/devices/disk/boot/@order?), or this patch should be reverted.
> Comments?

We should support the 'bootable' flag regardless. We gained an explicit

      <boot order='1'/>

in the <disk> element, which could easily be wired up to Xen's boot order
flag.

Daniel
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