Good evening, I have been pouring over an interesting problem and it seems like I need some deep insight. Windows Server 2025 Core has much more than expected overhead.

I am testing Windows Server 2025 Core (and Desktop) on Proxmox 8.3, with all paravirtualized features enabled and the agent installed.

On the VM all virt-io drivers have been installed and VBS has been explicitly disabled. The OS is newly installed, is completely idle, with no software installed, and running idle for quite some time, so no background or startup tasks.

I have tested various combinations of cpu flags, and kvm arguments, both with and without HPET, on two seperate hosts, both proxmox production and test, both Kernel 6.8 and Kernel 6.11. However, try as I might, over the course of three days, with various combinations of settings, the windows VM is consuming a lot of the hosts resources, a scenario not expected when comparing host cpu consumption on Hyper-V or VMware. All the while the VM performs very, very well, snappy with very very good IO performance, but the host efficiency is clearly not as expected.

When analyzing perf, kvm does have a notable amount of VM exits, its difficult to assess the scale of the consumption versus the host resources and the allocated resources.

What am I doing wrong here, I have tried everything under the sun, but idle resource consumption is far greater than expected.

I will test all ideas good and bad, i'm committed to making this efficient at any cost to my own personal time investments. I start a new role as IT Manager of a robotics company in a few weeks. Proxmox is what I want to use as my primary solution for all containers and VM across the enterprise, both windows and linux, I expect to scale, but this level efficiency causes problems with virtualization density.

Thank you all, I appreciate your time.

Virtual hardware:

Options

Guest performance:

I appreciate the feedback and your assertion is valid, thank you. While I am still learning exactly how to meter the performance differences between Server 2022 and Server 2025 on Proxmox, I think there is still something to uncover. Without more evidence its difficult to say much about whats occuring here, but my suspicion is that the hypercalls on Server 2025, potentially Windows 11 are not as well supported with KVM enlightenments as the older versions.

If anyone has feedback on how I can collect and share the kvm performance statistics in a meaningful way, Im happy to oblige.

The screenshot below shows Windows Server 2025 is running with more overhead on Proxmox.

This screenshot compares VM Exits between Server 2022 and Server 2025, which begins to show some differences in how the os is operating on KVM/Proxmox. Both are server core operating systems provisioned using the same servicing scripts to integrate VirtIO drivers and automate the installation of the Guest Agent. Both have explicitly been configured not to use VBS.

Server 2025

agent: 1
balloon: 0
bios: ovmf
boot: order=virtio0
cores: 2
cpu: host,flags=-md-clear;+pcid;-spec-ctrl;-ssbd;-ibpb;-virt-ssbd;-amd-ssbd;-amd-no-ssb;+pdpe1gb;+hv-tlbflush;-hv-evmcs;+aes
efidisk0: data:vm-101-disk-0,efitype=4m,pre-enrolled-keys=1,size=1M
hotplug: 0
machine: pc-q35-9.2
memory: 16384
meta: creation-qemu=9.2.0,ctime=1740872013
name: svr2025
net0: virtio=BC:24:11:AF:45:58,bridge=vmbr0
numa: 1
ostype: win11
scsihw: virtio-scsi-single
smbios1: uuid=057c4e97-4b43-4dfe-88d7-bbe9c6b456a5
sockets: 1
tablet: 0
vga: qxl
virtio0: data:vm-101-disk-2,backup=0,cache=unsafe,discard=on,iothread=1,replicate=0,size=128G
vmgenid: c2aec63c-436e-43ac-9b7a-4f0b3a36cf45

Server 2022

gent: 1
balloon: 0
bios: ovmf
boot: order=virtio0;net0
cores: 1
cpu: host,flags=-md-clear;+pcid;-spec-ctrl;-ssbd;-ibpb;-virt-ssbd;-amd-ssbd;-amd-no-ssb;+pdpe1gb;+hv-tlbflush;-hv-evmcs;+aes
efidisk0: data:vm-102-disk-0,efitype=4m,size=1M
hotplug: 0
machine: pc-q35-9.2
memory: 16384
meta: creation-qemu=9.2.0,ctime=1741910037
name: svr2022
net0: virtio=BC:24:11:6C:29:CE,bridge=vmbr0
numa: 1
ostype: win11
scsihw: virtio-scsi-single
smbios1: uuid=7ac22493-c5b2-45c4-90a6-4c4ebf9c1c2c
sockets: 2
tablet: 0
vga: qxl
virtio0: data:vm-102-disk-1,backup=0,cache=unsafe,discard=on,iothread=1,replicate=0,size=32G
vmgenid: be456a47-5e0f-4d81-bd47-26949d468d7f

Hmm, after continuing to dig deeper, I made further optimizations to virtual machines in an attempt to flush out the inefficiency. Clearly from these results, Windows Server 2025 is causing excessive interrupts when running under kvm \ proxmox, I suspect that the hv elightments for Windows Server 2025 is not fully mature.

I modified the virtual machine configuration (pve-q35-4.0.cfg) to remove all USB controllers.
I modified the virtual machine configuration (pve-q35-4.0.cfg) to remove all ICH9 PCI-E Root ports.
On the guests I disabled all unused devices.
On the guests I disabled all unused services.
On the guests I disabled all unused features.
On the guests, I have test various power profiles, and power settings.
On the guests, I have disabled Virtualization Based Security.
On the guests, I have installed all Virtual-IO-0.1.248 drivers, all devices are virtio.
Both guest operating systems are in exactly the same configuration state.

Windows Server Core, 2022 vs 2025

The full device tree can be seen here as read in a full desktop instance of Server 2025

I'm not crazy right!? Something is wrong here, any feedback is welcome.

It is a new Microsoft OS, it is going to be hot garbage until SP2. You’re running on a 8th gen Intel consumer CPU as well, the CPU flags you’re setting such as spec-ctrl aren’t on your CPU so they’re emulated. The remediations for speculative control are not enabled by default on 2022, but are on 2025. That’s just one of the many things that could be causing it. Disable (or enable) speculative mitigations on both sides.

I thought the same thing, both Server 2025 and 2022 are using v.266.

We are seeing much of the same behavior on one of our Hypervisors, 4-5% (of 8 CPUs) for a Windows Server 2025, that is idle, but we do see a much lower CPU-usage on another hypervisors. All is running with virtio .266-drivers.

Try:

sysctl kernel.numa_balancing=0

Hi

I also noticed degraded performance going from server 2022 to 2025.

If you instead of cpu model host use x86-64-v3 or something similar performance will be much better.
You can also disable nested virtualization and continue to use host as cpu model and performance is good again.

To disable nested virtualization on the host:
add kvm-intel.conf to /etc/modprobe.d/ containing

options kvm-intel nested=N

Don't forget to reboot after.

On the guests, I have disabled Virtualization Based Security.

Have you checked with msinfo32.exe ?

If you instead of cpu model host use x86-64-v3 or something similar performance will be much better.
You can also disable nested virtualization and continue to use host as cpu model and performance is good again.

I can confirm that using x86-64-v2 on our lab-setup (old hardware) made the suspicious idle-usage dropping by 50%

I cannot turn off nested virtualization, so someone else have to confirm that this is a factor. Is not possible to disable something on the guest to prevent the VM in question to use nestet virtualization in any shape or form?