Using PvGrub2 on Debian
Recently I wanted to move my Xen domU over to pvgrub2, the grand old bootloader ported to Xen. It shouldn't be hard, but as I am using btrfs on my domU with a /boot inside a /system subvolume, I'm making it a bit harder for myself.
Configuration on the domU
- Check that you have a kernel installed. In Debian, this usually requires the
linux-image-amd64package. You may already have one or more kernel packages installed, but this
linux-image-<arch>package is required for (security) updates.
- Install the
- Only if you have /boot in a non-standard location: make sure there is a
bootsymlink in the root of your filesystem to the correct boot directory, in my case
There may be other things you need to configure. For example, you may need to install
linux-headers-amd64) to make the initramfs updater happy. And when you're at it, you might want to remove old kernels left over from previous versions of Linux, if you have them.
Configuration on the dom0
- Make sure you have the
grub-xen-hostpackage installed. This package is most likely already installed as it is Recommended as part of Xen.
- Backup you guest configuration file!
- Adjust the guest configuration. I have a configuration like the following:
kernel = '/usr/lib/grub-xen/grub-x86_64-xen.bin'
root = ''
extra = '(xen/xvda2)/boot/grub/grub.cfg'
extra parameter must contain the full path to the
grub.cfg file, as GRUB sees it. If it's wrong, your guest will be stuck in a GRUB prompt (
grub>) and waste CPU cycles.
Note that the
root parameter has to be empty.
There are a few things that can go wrong, which will land you in a grub prompt. A great resource is the Ubuntu documentation for GRUB. But here are a few ideas.
If your guest doesn't seem to start and just wastes CPU, it is probably stuck on the command line. Start the guest with a console using
xl create -c /path/to/configfile.cfg, which should land you in a GRUB console.
If you are in the
grub> console, there are two things you can do to troubleshoot:
lsto see which root directories GRUB knows about. These are device names in parentheses. For me, these were (among others)
diskparameter in the guest configuration gives an indication which is which, but you can also put a path behind the device name to see what's in there (e.g.
(xen/xvda2)/boot). Find the grub.cfg file this way, try to
catit. The full path has to match the
extrapath parameter in the guest configuration. Edit it, shutdown (using
halt), and recreate the guest.
- If this still doesn't boot your guest, try to load the configuration using
source <path>. There may be an error while reading grub.cfg. In my case, it tried to load a file from
/bootwhich didn't exist on my system (possibly a bug in GRUB).