Urgent: NetBSD domUs will not start after Xen kernel update

[Stephen Borrill <netbsd%precedence.co.uk@localhost>]

I put XenServer hotfix XS72E008 onto a XenServer dom0 and was shocked to 
find that my NetBSD amd64/7 VMs will no longer boot. The VM briefly 
starts, but then dies before any console output. The hotfix updates the 
xenkernel from 4.7.2-2.2 to 4.7.2-2.6

https://support.citrix.com/article/CTX228722

Unfortunately, I'm kind of screwed here as there's no official way to 
revert hotfixes on XenServer.

Here's a working host:

host                   : xenserver-4
release                : 4.4.0+10
version                : #1 SMP Thu May 11 16:35:25 UTC 2017
machine                : x86_64
nr_cpus                : 24
max_cpu_id             : 23
nr_nodes               : 4
cores_per_socket       : 6
threads_per_core       : 1
cpu_mhz                : 2900
hw_caps                : b7ebfbff:17bee3ff:2c100800:00000001:00000001:00000000:00000000:00000100
virt_caps              : hvm hvm_directio
total_memory           : 65382
free_memory            : 37199
sharing_freed_memory   : 0
sharing_used_memory    : 0
outstanding_claims     : 0
free_cpus              : 0
xen_major              : 4
xen_minor              : 7
xen_extra              : .2-2.2
xen_version            : 4.7.2-2.2
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          : credit
xen_pagesize           : 4096
platform_params        : virt_start=0xffff800000000000
xen_changeset          : 3905d1e5f274, pq 619db0b419d5
xen_commandline        : dom0_mem=4096M,max:4096M watchdog ucode=scan dom0_max_vcpus=16 crashkernel=128M@256M console=vga vga=mode-0x0311
cc_compiler            : gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-4)
cc_compile_by          : mockbuild
cc_compile_domain      : [unknown]
cc_compile_date        : Thu May 11 16:35:43 UTC 2017
build_id               : ec22c432da2d73c26daec8625cb5a5f408963437
xend_config_format     : 4

[root@xenserver-1 ~]# rpm -ql xen-hypervisor
/boot/xen-4.7.2-2.2-d.config
/boot/xen-4.7.2-2.2-d.gz
/boot/xen-4.7.2-2.2-d.map
/boot/xen-4.7.2-2.2.config
/boot/xen-4.7.2-2.2.gz
/boot/xen-4.7.2-2.2.map
/etc/sysconfig/kernel-xen
/etc/sysconfig/kernel-xen-args

And the non-working one:

host                   : xenserver-1
release                : 4.4.0+10
version                : #1 SMP Thu Aug 10 20:29:14 UTC 2017
machine                : x86_64
nr_cpus                : 24
max_cpu_id             : 23
nr_nodes               : 4
cores_per_socket       : 6
threads_per_core       : 1
cpu_mhz                : 2900
hw_caps                : b7ebfbff:17bee3ff:2c100800:00000001:00000001:00000000:00000000:00000100
virt_caps              : hvm hvm_directio
total_memory           : 65382
free_memory            : 10360
sharing_freed_memory   : 0
sharing_used_memory    : 0
outstanding_claims     : 0
free_cpus              : 0
xen_major              : 4
xen_minor              : 7
xen_extra              : .2-2.6
xen_version            : 4.7.2-2.6
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          : credit
xen_pagesize           : 4096
platform_params        : virt_start=0xffff800000000000
xen_changeset          : 3905d1e5f274, pq c5caa7a13f33
xen_commandline        : dom0_mem=4096M,max:4096M watchdog ucode=scan dom0_max_vcpus=16 crashkernel=128M@256M console=vga vga=mode-0x0311
cc_compiler            : gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-4)
cc_compile_by          : mockbuild
cc_compile_domain      : [unknown]
cc_compile_date        : Mon Oct  9 14:52:07 UTC 2017
build_id               : 1c98b4369cd63ca966ab8ec1bbd057e366851eaf
xend_config_format     : 4

[root@xenserver-1 ~]# rpm -ql xen-hypervisor
/boot/xen-4.7.2-2.6-d.config
/boot/xen-4.7.2-2.6-d.gz
/boot/xen-4.7.2-2.6-d.map
/boot/xen-4.7.2-2.6.config
/boot/xen-4.7.2-2.6.gz
/boot/xen-4.7.2-2.6.map
/etc/sysconfig/kernel-xen
/etc/sysconfig/kernel-xen-args

--
Stephen