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

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

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


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