(UEFI) Booting Xen from a btrfs snapshot fails

This document (000020602) is provided subject to the disclaimer at the end of this document.

Environment

SUSE Linux Enterprise Server 15
SUSE Linux Enterprise Server 12

Situation

Using a BTRFS snapshot in GRUB2 to boot a Xen image fails, but booting standard Linux kernel works.

One can see errors similar to:

Resolution

Engineering is aware of the issue. To work around the boot issue, boot to a non-Xen image boot entry (a standard Linux kernel), and rollback/reinstall old Xen packages as needed, like this EXAMPLE:

# zypper in --allow-downgrade --oldpackage xen-4.14.3_04-3.15.1

Cause

There are multiple issues affecting the Xen image booting using UEFI.

First, grub2-mkconfig copies xen-<version>.efi (and other files) into /boot/efi/efi/sles/
/boot/efi is the EFI partition, which is VFAT formatted. On VFAT no BTRFS snapshots are possible, means booting is not possible if Xen should boot from an older snapshot than the current as the xen.efi loader must match the currently installed xen version.

Reason: the Xen image is currently booted via Xen EFI loader

# grep -m1 -A 1 'Loading Xen' /boot/grub2/grub.cfg 
        echo    'Loading Xen 4.14.3_06-150300.3.18 with Linux 5.3.18-150300.59.49-default ...'
        chainloader  /efi/sles/xen-4.14.3_06-150300.3.18.efi xen-4.14.3_06-150300.3.18.efi  config.1

However, the current snapshot boot implementation is based on passing "rootflags=subvol=/@/.snapshots/<id>/snapshot" kernel boot parameter, and there is no way to make Xen.efi loader use this parameter from GRUB2.

Status

Reported to Engineering

Disclaimer

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