--- Comment #6 from James Carter firstname.lastname@example.org --- "Good" news from a test: I have these two identical machines, one with a VM and one without; the VM host freezes reliably and the leaf node has never been seen to freeze. OK, on the leaf note (Iris) I installed qemu-kvm-5.0.0-3.1.x86_64 libvirt-daemon-6.4.0-1.1.x86_64 and numerous friends (the latest) and created a new VM called Orion, basically copying the XML and software selection from my other VMs. For testing I ran a console session on the guest (VNC through virt-viewer) and set the guest's screensaver to show random eye candy, giving the guest and host consistent but not excessive net traffic and CPU load. (I have the impression that the eye candy is not necessary; just being booted is enough; but that's not assured and it's more prudent to load up the guest, to maybe avoid a false negative.)
With kernel 5.6.12 on the host and 5.7.1 on the guest, it ran for 14 hours without freezing. With 5.7.1 on both the host and the guest, it ran for 5 hours and the host froze up, being rebooted by its watchdog timer. I can't tell if the guest had any issues just before the host froze; I was in bed at the time. As usual, the log files on both the host and guest look perfectly normal, then end with a block of \0, and continue on with normal-looking output from the rebooted session.
So this does support the hypothesis that machines running kernel 5.6.14+ (here, 5.7.1) will freeze if and only if they are host to a VM. It does not support the alternative fact that the recent update to libvirt and qemu may have made the host stop freezing. In the upgrades to 5.6.14 and kernel-default-5.7.0-3.1.gad96a07.x86_64 two of the VM hosts would freeze within 5 minutes of booting (tried several times each), but hosts that are less mission critical and more convenient for testing also take a long time to freeze, if they're going to.
It's going to be hard to get developers to pay attention to this problem if they can't reproduce it in-house, and I'm going to try to put together an easily describable software and VM configuration that will freeze up.
@tiwai, I retrieved kernel-default-5.6.12-1.3.x86_64 from http://download.opensuse.org/history/ , thanks for the link!