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

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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.

** Affects: grub2 (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: grub root rpool xen zfs
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ubuntu 20.04.1 installed with zfs root fails to boot after installing xen-hypervisor
https://bugs.launchpad.net/bugs/1894101
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