[GLLUG] Multiple grub installs in xen guest

[Tim Woodall]

On Fri, 11 Oct 2019, Tim Woodall via GLLUG wrote:

> Does anyone know whether the default debian grub-xen stuff allows you to
> select the grub root?
>
> I have a bootable xen image that uses grub from /dev/xvda2. I also have
> one that uses grub from /dev/vg-mirror/boot. (the vg is inside the disk
> I export to the guest, not the vg of dom0)
>
> I want to be able to boot the former, but have the latter available to
> mount. But it is insistent that it will use the vg boot in favour of the
> raw ext3 one.
>
> I've tried extra="(xen/xvda2)" and extra="root=(xen/xvda2)" but neither
> seem to make a difference and it persists in using lvm/vg--mirror-boot.
>
> This weekend I'll have a look at the source for the grub-x86_64-xen.bin
> bootloader and tweak it if necessary but perhaps someone already knows
> how to do this?
>
Looks like the extra parameter is completely ignored :-(  There appears
to be nothing in grub that is passed from xen.

The debian bootloader does the standard look for pvgrub in /boot/xen,
then /xen, and then fall back to /boot/grub/grub.cfg and then
/grub/grub.cfg using the search command.

I've built a trivial replacement for grub-x86_64-xen.bin that does the
right thing for my particular case. Just a tiny tad annoying that there
doesn't seem to be any other way to do this than build a specific file
for each guest if there's more than one possible grub to boot.

Tim.