High VM-EXIT and Host CPU usage on idle with Windows Server 2025

[pm_username2025]

I've tried to migrate from 2 Windows Server 2025 Core Hyper-V VMs running on Win10 2019 LTSC IoT and Dell E7470 with i5-6300u (2C/4T), 32 GB of RAM, NVMe SSD. The laptop has cooper radiators on SSD, RAM sticks, PTM 7950 on CPU die and it is lifted few centimeters above desk for better air cooling. When Hyper-V is utilized, the cooler starts spinning only on heavy tasks like Windows Updates installation, restarts. Windows host CPU utilization barely reached 3-5% during idle remote session on Hyper-V host.

I converted vhdx images to QCOW2 and created Proxmox VMs. The same VMs running on Proxmox VE utilized int total 4-8% of the CPU. Fan was spinning all the time. Windows VirtIO Drivers were installed. I've tried various Proxmox settings but I gave up few hours later and restored Windows 10 from backup.



I am afraid we may proceed with Win Sever 2022 instead of 2025 on new infra just for this reason

 

[SchorschSK]

Ok so hyper-v also does not show any degradation and has the same baseline (between 23h2 and 24h2).

As I think we have collectively tried out every parameter, it is likely that the issue will be a core Qemu issue.

We should try out with vanilla Qemu to see if Proxmox might for example have missed a patch or the problem is in upstream Qemu.

 

[uberarchiver]

https://www.phoronix.com/news/Linux-6.18-KVM



And google : "kvm enable avic disable nested virtualization"

Might help. Might not help. Got some VMEXITs eliminated myself

 

[SchorschSK]

https://www.phoronix.com/news/Linux-6.18-KVM

Ok this sounds promising. Did you try out 6.18 or did you disable nesting and enable avic? Intel or AMD cpu? I tried enabling avic but might have missed the part with disabling nesting so gonna try it again when time allows.

 

[olegfusion]

Any news about this issue? Using X86-64-v2-aes, kvm inside kvm nested virtualization, win11 25h2 - 20% idle cpu usage.

 

[Johannes S]

On modern systems there are also other CPU types you can try, you can determine the best possible option with https://github.com/credativ/ProxCLMC

 

[ChrisBozeman]

I came here today to tag @RoCE-geek. I know they were working on this.

But I also want to say, "There shouldn't be a need to experiment with CPU types". If you have hardware from the past 10-12 years, it should just work with selecting "host" as the CPU type. I have a fairly large homelab cluster of three R640s all with 384 GB of DDR4 and dual Intel Xeon Platinum 8168 CPUs and an older R730xd with dual Intel Xeon E5-2696v4 CPUs.

I recently took one of the R640s out of the cluster and installed Windows Server 2025 on it to act as a Hyper-V host. This is completely a Proxmox issue. The idle CPU issue does not exist there.

And before someone argues semantics like, "It's not actually a Proxmox issue, it's a Linux kernel issue!" or "It's actually a Windows Server 2025 issue!"

All of that may be true. None of it matters.

I cannot recommend Proxmox as a replacement for VMware to my clients if they're going to see incredibly poor performance.