grub-common: generated xen boot entries incorrect with root on zfs

[James Dingwall]

I have a system with a ZFS root (/) and which boots xen. The generated grub.cfg kernel command line is ok for a non-xen boot but is missing the pool name from the xen entries. Since upgrading from 18.04 it is necessary to modify the command line via the grub editor during boot or edit grub.cfg directly.

i.e. direct linux looks like:

linux /vmlinuz-5.4.0-40-generic root=ZFS=rpool/ROOT/ubuntu ro

and xen entries look like:

module2 /vmlinuz-5.4.0-33-generic placeholder root=ZFS=/ROOT/ubuntu ro console=hvc0 console=tty0 earlyprintk=xen

grub's 10_linux includes this block:

        # We have a more specialized ZFS handler, with multiple system in 10_linux_zfs.
        if [ -e "`dirname $(readlink -f $0)`/10_linux_zfs" ]; then
          exit 0
        fi

but no similar provision for systems booting xen has been made.

# lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 20.04 LTS
Release: 20.04
Codename: focal

# apt-cache policy grub-common
grub-common:
  Installed: 2.04-1ubuntu26
  Candidate: 2.04-1ubuntu26
  Version table:
 *** 2.04-1ubuntu26 500
        500 http://gb.archive.ubuntu.com/ubuntu focal/main amd64 Packages
        100 /var/lib/dpkg/status