Public bug reported:
I upgraded a KVM host from Ubuntu 24.04 to Ubuntu 26.04. After the
upgrade, none of my libvirt VMs would start because their domain
definitions used `pc-q35-4.2`, which is unavailable in the target QEMU.
One of these VMs provided DNS and DHCP for my home network, so losing
all guests caused a significant outage. `virsh start` gave the error
`Emulator '/usr/bin/qemu-system-x86_64' does not support machine type
'pc-q35-4.2'` (or similar - I don't have the exact text).
The VMs were recovered by editing their domain XML and changing the
machine type to `pc-q35-10.2`.
My strong recommendation is that the upgrade process should check
libvirt domain definitions against the machine types available in the
target QEMU version and show an actionable warning before proceeding
when a configured type will no longer be available. Since changing a
machine type can alter guest-visible hardware, an automatic rewrite may
be unsafe; a pre-upgrade warning that lets the administrator postpone
the upgrade and migrate/test the VMs would prevent this avoidable
outage.
**Expected:** Before the host upgrade, identify affected VMs and explain
that their configured machine type will be unavailable after upgrading,
with guidance to back up and move them to a supported type.
**Actual:** The host upgrade completed, but all affected VMs failed to
start afterward.
**Environment:** Ubuntu 24.04 → 26.04 KVM host; affected guests
configured with `pc-q35-4.2`; target QEMU 10.2.
**Workaround:** Change the machine type in each VM’s libvirt domain XML
to a type supported by the upgraded host.
```
mh@mh-nas:~$ lsb_release -rd
Description: Ubuntu 26.04.1 LTS
Release: 26.04
mh@mh-nas:~$
mh@mh-nas:~$ apt-cache policy qemu-system-x86 libvirt-daemon-system
ubuntu-release-upgrader-core
qemu-system-x86:
Installed: 1:10.2.1+ds-1ubuntu3.3
Candidate: 1:10.2.1+ds-1ubuntu3.3
Version table:
*** 1:10.2.1+ds-1ubuntu3.3 500 (phased 20%)
500 http://archive.ubuntu.com/ubuntu resolute-updates/main amd64
Packages
100 /var/lib/dpkg/status
1:10.2.1+ds-1ubuntu3 500
500 http://archive.ubuntu.com/ubuntu resolute/main amd64 Packages
libvirt-daemon-system:
Installed: 12.0.0-1ubuntu5.6
Candidate: 12.0.0-1ubuntu5.6
Version table:
*** 12.0.0-1ubuntu5.6 500 (phased 20%)
500 http://archive.ubuntu.com/ubuntu resolute-updates/main amd64
Packages
100 /var/lib/dpkg/status
12.0.0-1ubuntu5.5 500
500 http://security.ubuntu.com/ubuntu resolute-security/main amd64
Packages
12.0.0-1ubuntu5 500
500 http://archive.ubuntu.com/ubuntu resolute/main amd64 Packages
ubuntu-release-upgrader-core:
Installed: 1:26.04.27
Candidate: 1:26.04.27
Version table:
*** 1:26.04.27 500
500 http://archive.ubuntu.com/ubuntu resolute-updates/main amd64
Packages
500 http://archive.ubuntu.com/ubuntu resolute-updates/main i386 Packages
100 /var/lib/dpkg/status
1:26.04.18 500
500 http://archive.ubuntu.com/ubuntu resolute/main amd64 Packages
500 http://archive.ubuntu.com/ubuntu resolute/main i386 Packages
mh@mh-nas:~$
```
** Affects: ubuntu-release-upgrader (Ubuntu)
Importance: Undecided
Status: New
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https://bugs.launchpad.net/bugs/2172127
Title:
Release upgrade should warn when libvirt guests use machine types
unavailable in the target QEMU
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