Welcome to the forum, and thanks for writing this up instead of just fixing it on your own box. You already found the thread I was going to point you at. You're ahead of me there. What happened in it after the part you read: Olivier bisected the problem down to Linux 6.12.5, and Teddy linked an upstream fix that landed in tip (f24df84cbe05), with no ETA for when distros pick it up. The tsc_mode=2 plus nomigrate workaround came out of that, and it carries the live migration cost you already spotted. One thing I'd like to know, and I don't think anyone has tried it on your side. Does the leftover couple of seconds per vCPU go away if you set tsc_mode=2 on one of those VMs? If it does, that would tie your case to the same regression rather than to a second thing. I'm not deep enough in the timer code to tell you whether the ttyS0 amplification and the TSC regression are the same mechanism or two things stacking. Probably worth a mention to @Team-Hypervisor-Kernel so someone who is can have a look.

@furyflash777 I'm assuming you're on Ubuntu? Indeed as Olivier said this is tested on Debian and doesn't cause issues, but it seems on the newer Ubuntu versions with cloud-init, the new Netplan based network manager and how it interacts with cloud-init breaks/gets wiped if no cloud-init drive is found. Yet another cloud-init bug to track down