[libvirt] [PATCH] xen: fix PyGrub device order using boot/@order

[Philipp Hahn]

Hello Kiri,

Am Montag 31 Oktober 2011 15:15:12 schrieb Jiri Denemark:
> Is there any way you can pass the exact order to Xen?

Not that I know of. This is one of the corner cases, which is only relevant if 
you have a Xen-PV domain AND are using PyGrub as the boot-loader. For HV the 
flag is unused and for direct kernel boot you don't need it either. Only for 
PV+bootloader needs Xend a way to find the disk containing the kernel.

In xen-4.1/tools/python/xen/xend/XendDomainInfo.py:3234 _configureBootloader() 
build a list of all vbds marked bootable and passes only the first one to 
Pygrub. Technically you mark multiple disks as bootable, but the flag is only 
set in xen-4.1/tools/python/xen/xend/XendConfig.py:1498 device_add(), which 
does this exactly once for the first disk added.

> Multiple devices with 
> bootIndex are allowed (the indexes are unique of course) to specify booting
> preference and by setting (bootable 1) anytime bootIndex != 0 makes all
> such devices bootable in unspecified order.

Same with Xen — I thinks: If you mark multiple disks as bootable, the first 
disk gets used — whichever that is.

> If only one bootable device is 
> supported by Xen, you should probably emit (bootable 1) only for bootIndex
> = 1 and maybe even forbid more than one device with bootIndex.

This makes perfect sense for libvirt → xen, since we want determinictic 
behaviour.
But if multiple disk are marked bootable, converting xen → libvirtd either has 
to pick one disk and report just that disk as bootable; or reports all disks 
marked as bootable.
Either way, round-trip is broken. I wish XenLight would be useable.

> You also need to add 'deviceboot' feature to guest capabilities XML so that
> apps know they can use <boot order='n'/> when creating this kind of guests.

Will have a look.

Sincerely
Philipp
-- 
Philipp Hahn           Open Source Software Engineer      hahn at univention.de
Univention GmbH        Linux for Your Business        fon: +49 421 22 232- 0
Mary-Somerville-Str.1  D-28359 Bremen                 fax: +49 421 22 232-99
                                                   http://www.univention.de/
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