So, I have a client that has an internal use application that needs an ancient version of libc5. That's not a typo; libc5. Before the server that ran it died about a year and a half ago (said server was an AMD K6-2/450 with a 6GB Western Digital Caviar drive that had been spinning nearly continuously for almost 20 years!) it was running on Red Hat Linux 5.2. The last version of CentOS that shipped with a libc5 was 2.1. So I set them up a KVM guest running CentOS 2.1, mainly because Red Hat 5.2 wouldn't recognize the network at all, using the e1000 network driver, and it ran well.
Now, yes, there are no updates, but no it doesn't matter; it's an internal use application that has a very small footprint and very low risk.
After the update on the host to 7.7.1908, the network stopped running. The host also has a CentOS 7 guest that is still working properly. If I change the 2.1 system to not automatically load the e1000 driver and console in and 'modprobe e1000' manually, it starts working again, for a while.
Any ideas as to where to start looking? qemu-kvm-ev itself, or libvirt? I'm planning to start out with rollbacks to the previous versions of each to try to find where the issue starts.