[Bug 1894101] Re: ubuntu 20.04.1 installed with zfs root fails to boot after installing xen-hypervisor

[Todd]

I have two different Ubuntu 20.04.1 systems using ZFS on root. Only one
has Xen installed, but both have had this same problem with rpool
missing from the grub.cfg and fail to boot after update-grub/grub-
mkconfig is run. Both systems share an error during /etc/grub.d/30_os-
prober:

----- System one ----
### BEGIN /etc/grub.d/30_os-prober ###
device-mapper: reload ioctl on osprober-linux-nvme0n1p4  failed: Device or resource busy
Command failed.
### END /etc/grub.d/30_os-prober ###

----- System two ----
### BEGIN /etc/grub.d/30_os-prober ###
device-mapper: reload ioctl on osprober-linux-sda7  failed: Device or resource busy
Command failed.
### END /etc/grub.d/30_os-prober ###

I have uploaded the full log.


** Attachment added: "Log of grub-mkconfig"
   https://bugs.launchpad.net/ubuntu/+source/grub2/+bug/1894101/+attachment/5440632/+files/grub-mkconfig.log

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https://bugs.launchpad.net/bugs/1894101

Title:
  ubuntu 20.04.1 installed with zfs root fails to boot after installing
  xen-hypervisor

Status in grub2 package in Ubuntu:
  New

Bug description:
  After installing 20.04.1 using the ZFS root option I installed Xen.
  When it rebooted it failed in initramfs with the error "No pool
  imported". I was able to boot the previous kernel. When I examined the
  /boot/grub/grub.cfg I found what looked like invalid "root=ZFS="
  options like this:

  "root=ZFS=/ROOT/ubuntu_xxxxxx"

  The previous kernel that boots looks like this:

  "root=ZFS=rpool/ROOT/ubuntu_xxxxxx"

  After looking in /etc/default/grub and /etc/grub.d I was unable to
  figure out why they were different and how I could change it.

  As a test saved a copy and edited /boot/grub/grub.cfg adding the rpool
  to each "root=ZFS=" line where rpool was missing. This time I was able
  to boot the Xen kernel.

  ==============================
  How to reproduce the problem

  From scratch install ubuntu 20.04.1 desktop, select ZFS as root option, then install xen-hypervisor-4.11-amd64.
  It will fail to reboot because the grub root=ZFS argument is wrong.

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