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

[Todd]

In line 137 of file "10_linux":

rpool=`${grub_probe} --device ${GRUB_DEVICE} --target=fs_label
2>/dev/null || true`

The command run returns this error:
root at stream:/etc/grub.d# /usr/sbin/grub-probe --device /dev/nvme0n1p4 --target=fs_label
/usr/sbin/grub-probe: error: unknown filesystem.

The result makes ${rpool} NULL, and sets ${LINUX_ROOT_DEVICE} to
"ZFS=/ROOT/ubuntu_aaodxz"

This is clearly wrong and looks like the root cause of this issue.

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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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