Illegal instruction when running in kvm?

[Libor Klepáč <libor.klepac@bcom.cz>]

Hello,

i have Debian 11 desktop VM with KDE running on Debian 11 machine using
KVM.
I'm migrating from current Dell T1700SFF with i7 CPU to Dell R420 with
xeon processor.

I used dd to transfer disks from old machine to new and then copied
it's vm.xml from libvirt.

When i launch desktop on new server, i get this

Executable: ksplashqml PID: 2146 Signal: Illegal instruction (4) Time:
2/11/22 10:12:49 AM CET

and dmesg says
[  248.820494] traps: QSGRenderThread[2145] trap invalid opcode
ip:7fd5842f6027 sp:7fd5858a0820 error:0


ksplash, plasmashell, systemsettings5 - all does not work, crashes with
illegal instruction.

But for example kwin, konsole, dolphin - all this is started in my KDE
session.

What can be the problem?
I checked filesystem on cloned machine with btrfs scrub, tried to
reinstall libc6 and all libqt5 packages, also run debsums to check all
files - everything was OK.

I also tried to upgrade libvirt (7.0 -> 8.0) and qemu (5.2 -> 6.2) on
my new server.


I use nomachine to connect to desktop.
Also tried xrdp.
Also connected via ssh and started systemsettings5, with the same error

Any clues?
Thanks,
Libor