Re: Xen vs. GRUB

["Ian C. Blenke" <ian@blenke.com>]

Samuel Thibault wrote:

>Thomas Schwinge, le Wed 03 Jan 2007 13:09:25 +0100, a écrit :
>  
>
>>Is there a consensus that GRUB (or rather GRUB2) should be ported to
>>allow them to run in a Xen environment?
>>    
>>
>
>I'm not sure grub on Xen would be so useful, since people usually choose
>their operating system and parameters directly from Xen configuration
>files.  And I actually already contacted Xen people, and they agree that
>adding a multiboot-like module abstraction to Xen won't be hard and
>would be useful.
>  
>

Xen PV domUs can boot from "pygrub". This is the method used by JailTime 
and rPath to encapsulate kernels and initrd inside the disk images.

So there is already such a "port". Just specify a bootloader in your xen 
config:

    bootloader="/usr/lib/xen/bin/pygrub"

and it's easy enough to override the config file with your own boot entry:

    bootentry="hda1:/boot/vmlinuz-xen,/boot/initrd-xen"

Which does seem a bit silly when you can specify the same thing with 
root=, ramdisk=, and anything else with extra=. Alternatively, there are 
libvirt and xen-tools and other xen config abstrations outside of xm.

Xen HVM domUs, on the other hand, bootstrap whatever MBR is installed on 
the virtual disk. No changes needed here at all.

The next step is HVM with paravirt_ops/VMI, which might actually show up 
some day for Xen (not there yet). Guests would still run in an HVM jail 
but are aware of the hypervisor and don't need to interact with the QEMU 
provided virtual hardware (think PV drivers).

Also, kvm has paravirt_ops now, as of a couple of days ago (thanks to 
Ingo). With l-hype and kvm, xen is quickly becoming moot anyway.

How does this affect grub and other bootstrapping? We very well might be 
talking about paravirt_ops/VMI and/or PV drivers at some point in 
addition to vanilla int13 real mode devices (though the virtual BIOSes 
will likely take care of this anyway).

- Ian C. Blenke <ian@blenke.com>