Re: [Xen-devel] Xen 4.x / Linux 3.x (dom0 and HVM domU) and NIC handling

[Alex Bligh <alex@xxxxxxxxxxx>]

Ian,

--On 2 December 2011 17:42:35 +0000 Ian Campbell <Ian.Campbell@xxxxxxxxxx> 
wrote:
> Right.
>
> > FWIW my experience is that various built-for-cloud type distros don't use
> > that scheme, mainly because they use grub1 which IIRC does not support
> > this, and building images in a non-root environment that have grub1
> > in is rather easier than grub2.
> UUID= and LABEL= are functions of your initrd (actually udev) and not
> the bootloader. They work with both grub1 and grub2.
I /think/ we might be slightly at cross purposes.

At least when I was busy making images for Xen for PVHVM a couple of
years ago, the problem is roughly as follows:

The boot loader needs to know what device to load the kernel and initrd 
from. To do this (roughly speaking) it needs to know what BIOS device to 
use. Of course it does not matter whether the boot loader uses the PV 
device or the emulated device, save that this requires the emulated device 
be present (at least whilst the boot loader doesn't support drivers for the 
pv device). The problem is that the device the boot loader accesses is in 
general specified in the boot loader configuration file not as a bios 
device number, but as a device name. Equally, it needs to know the device 
so that it can write the boot sector in order to reinstall the boot loader, 
set options etc. The problem we ran into here was that this needs to be set 
to xvda in order to to write the boot sector etc., because the sda device 
is unplugged. However, it only recognised a BIOS mapping for sda. So 
neither worked, without fiddling with mappings, but that wasn't possible on 
a straight install. For some reason, grub1 was far more forgiving.
> > So, for instance, all the vm-builder stuff in debian/ubuntu used grub1
> > and did not work this way.
> Which ones are these?
EG:
http://packages.ubuntu.com/search?keywords=ubuntu-vm-builder
http://wiki.debian.org/VMBuilder

> The Debian installer uses UUID where it can AFAIK. Ubuntu's installer is
> derived from Debian's so I'd expect it to be the same.
There is more than one method of generating debian/ubuntu images
(debootstrap, multistrap, vmbuilder to name but 3). Some of these
run an installer, some just generate a working image for a particular
environment (and it's the latter which are problematic).

FWIW my understanding is that though Ubuntu and Debian's installers both 
use the same underlying d-i stuff (or used to), they are now reasonably 
different (not that this has much bearing on the argument, as the main 
difference between the two is the kernel which has led to rather different 
results between the two); certainly which boot loader they use is
independent of the install architecture, their partitioning schemes
have been substantially different, and I would expect use of UUID/LABEL
not necessarily to correspond.

--
Alex Bligh

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