Re: [Xen-users] how to boot a para-virtulized vm from cdrom after installation

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2012-10-09 at 15:41 +0100, Nick Couchman wrote:
> >>> On 2012/10/08 at 10:08, Alexandre Kouznetsov <alk@xxxxxxxxxx> wrote:
> 
> > Hello.
> > 
> > El 06/10/12 08:38, jun hu escribiÃ:
> >> how to boot a para-virtulized vm from cdrom after installation?
> > 
> > In short, it does not work that way.
> > 
> > When you decide "where to boot from", it implies there is some piece
> of 
> > code (BIOS, for example) that will look for a bootloader on some
> device. 
> > This is perfectly valid in HVM mode of operation, but not in PV.
> > 
> > In case of Para-Virtualized DomU, the hypervisor itself loads the 
> > kernel, specified in the config file. There is no BIOS or bootloader
> 
> > residing in the MBR of a block device. There is no no such thing as 
> > "boot device" or "boot order".
> > 
> > You can try this:
> > 1. Hard way. Set up a new, completely new, HVM DomU. Attach your
> former 
> > PV DomU to it and boot form CD.
> > 
> > 2. Depends on what you need to achieve over your DomU, it almost 
> > certainly possible to be done from Dom0.
> > 
> 
> It's worth noting that some of the Xen management tools, like
> virt-manager (open source Xen) and XenCenter (XenServer/XCP) try to help
> you with this process.  If you choose to boot one of these from a CD,
> the management tool goes out and looks in common locations for a
> Xen-based kernel, copies the kernel and initrd over to a temp location,
> and boots the VM in PV mode with the CD/DVD attached.

This is the "eliloader" tool, it has a pygrub like interface. 

>   This makes the
> process transparent to the end user, but it still is not truly booting
> off of a CD.  Most modern Linux distributions have a Xen kernel and
> initrd located somewhere on the install media for just this purpose.

There's an example of how to use pygrub for this purpose at
http://wiki.xen.org/wiki/Debian_Guest_Installation_Using_Debian_Installer#Create_Initial_Guest_Configuration_2

pygrub actually speaks isolinux so if the kernels referenced by the
isolinux config on the CD are xen capable then things might just work...

Ian.


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