This patch has seen about 10 hours of testing on an 13-CURRENT host with 12.1-RELEASE and Windows 10 1909 guests. Host CPUs were a Core i7-2620M, a Core i7-6700k and a Xeon Silver 4110. I'll deploy it to several production servers when I'm back in the office in early january. It might be good idea to do some testing on Linux guests.
I've run the second version of this patch for one week on several production hosts, some of them with APICv support and without. Guests were Windows Server 2019, Windows Server 2012 R7, Windows 7, Ubuntu 18.04.3 and FreeBSD 12.1. I didn't observe any problems, both the hosts and guest were stable. Guest memory consumption was about the same as without the patch.