A third revision, incorporating all feedback received here and in private mail:

I have a very experimental patch that deactivates TPR shadowing if x2apic mode is used. I'll clean it up and update the review sometimes next week.

Here's a second version, incorporating feedback received here and in private mail:

I have tested various host CPUs and Windows versions as a guest and do not see any increased memory consumption. I think it's misleading because Windows has much more CPU cycles available due to TPR shadowing and therefore fills up the RAM allocated to the guest faster. For example, on a Win 10 1909 VM with 2GB of RAM it takes about 20 minutes after boot (idle, without any activity) to reach a memory usage of 2GB (on the host, measured by the 'RES' column in top) without the patch, but only 4 minutes with the patch. With CDM it is similar. Without patch, it takes several runs, with patch, two are enough. But in the end the memory consumption is the same. Do you ever have the exact bhyve cmd for me? I could try to reproduce the problem with it.

Hi Michael,
thank you for your feedback. So far I haven't seen any increased memory consumption and there is nothing in the patch that could explain it. Unless it uncovers another bug somewhere in the Bhyve code. Or I'm overlooking something, of course. I'll look into it.