Comment 3 for bug 1956166

As Belgarath says, it takes xen-4.15+

tlviewer@xen2204:~$ sudo xl list
Name ID Mem VCPUs State Time(s)
Domain-0 0 600 2 r----- 45.8
ub2004 1 804 2 -b---- 10.7
ub2204 2 360 2 -b---- 9.2
fc34min 3 604 2 -b---- 22.8
al8pv 4 854 2 -b---- 13.4
fc35min 5 784 2 -b---- 28.3

sudo systemd-analyze time
Startup finished in 8.070s (kernel) + 32.310s (userspace) = 40.380s
multi-user.target reached after 32.271s in userspace

tlviewer@xen2204:~$ sudo xl info
host : xen2204
release : 5.13.0-19-generic
version : #19-Ubuntu SMP Thu Oct 7 21:58:00 UTC 2021
machine : x86_64
nr_cpus : 2
max_cpu_id : 3
nr_nodes : 1
cores_per_socket : 2
threads_per_core : 1
cpu_mhz : 3062.672
hw_caps : bfebfbff:0008e3bd:20100800:00000001:00000000:00000000:00000000:00000000
virt_caps : pv hvm shadow
total_memory : 4094
free_memory : 36
sharing_freed_memory : 0
sharing_used_memory : 0
outstanding_claims : 0
free_cpus : 0
xen_major : 4
xen_minor : 15
xen_extra : .1
xen_version : 4.15.1
xen_caps : xen-3.0-x86_64 xen-3.0-x86_32p hvm-3.0-x86_32 hvm-3.0-x86_32p hvm-3.0-x86_64
xen_scheduler : credit2
xen_pagesize : 4096
platform_params : virt_start=0xffff800000000000
xen_changeset : a763f8f158e81158b-x86/spec-ctrl: Fix default calculation of opt
xen_commandline : dom0_mem=600M,max:648M guest_loglvl=all loglvl=all sched=credit2 console=vga vga=gfx-1280x1024x16 conring_size=48k xsave=0 xpti=false cpufreq=xen:performance
cc_compiler : gcc (Ubuntu 11.2.0-13ubuntu1) 11.2.0
cc_compile_by : pbuser
cc_compile_domain : prymar56.org
cc_compile_date : Sat Jan 8 13:42:29 PST 2022
build_id : 3a9d3ef047996d4f6be41d9dadda2363d6538db7
xend_config_format : 4

tlviewer@xen2204:~$ sudo dpkg -l | grep -P 'xen|qemuu'
ii libxen-4.15:amd64 4.15.1-1+ub22u04.2 amd64 Public libs for Xen
ii libxenstore3.0:amd64 4.15.1-1+ub22u04.2 amd64 Xenstore communications library for Xen
ii libxentoolcore1:amd64 4.15.1-1+ub22u04.2 amd64 helper for qemu & libxenstore
ii qemuu 6.1.0+1-1+ub22u04.2 amd64 qemu-system-i386 (6.1.0/xen-4.15) with 9pfs support
ii xen-hypervisor-4.15-amd64 4.15.1-1+ub22u04.2 amd64 Xen Hypervisor on AMD64
ii xen-system-amd64 4.15.1-1+ub22u04.2 amd64 Xen System on AMD64 (meta-package)
ii xen-utils-4.15 4.15.1-1+ub22u04.2 amd64 XEN administrative tools
ii xen-utils-common 4.15.1-1+ub22u04.2 all Xen administrative tools - common files
ii xenstore-utils 4.15.1-1+ub22u04.2 amd64 Xenstore command line utilities for Xen

this is an official build using a delta similar to pre-Sep-2019 days applied to the full upstream tarball.