qemu-system-x86_64: Windows 11 24H2 and 25H2 idle while host CPU usage is high
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## Host environment
- Operating system: Debian
- OS/kernel version: 6.12.57+deb13-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.12.57-1
- Architecture: x86_64
- QEMU flavor: qemu-system-x86_64
- QEMU version: QEMU emulator version 10.0.6 (Debian 1:10.0.6+ds-0+deb13u2)
- QEMU command line:
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command line arguments in /var/log/libvirt/qemu/$GUEST.log.
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```
qemu-system-x86_64 -m 8192M -M q35 -smp 12 -accel kvm -cpu host,hv_stimer,hv_ipi,hv_runtime,hv_synic,hv_time,hv_relaxed,hv_spinlocks=0x1fff,hv_vapic,hv_vpindex \
-device virtio-scsi-pci,id=scsi \
-drive file=25h2.qcow2,format=qcow2,discard=unmap,if=virtio,aio=native,cache.direct=on,id=drive0 \
-drive if=pflash,file=RELEASEX64_OVMF_CODE.fd,format=raw \
-device uefi-vars-x64,jsonfile=nwq-windows-vars.json \
-netdev tap,helper=/usr/lib/qemu/qemu-bridge-helper,id=net0 \
-device virtio-net-pci,netdev=net0,id=nic1,mac=52:54:00:00:81:26 \
-device vmgenid,guid=3e81a66e-7b9d-49bc-bb20-037fcabcece4 \
-device virtio-balloon,free-page-reporting=on \
-monitor unix:./monitor-sock,server,nowait \
-qmp unix:./qmp-sock,server,wait=off \
-chardev socket,path=./qga.sock,server=on,wait=off,id=qga0 \
-device virtio-serial \
-device virtserialport,chardev=qga0,name=org.qemu.guest_agent.0 \
-display vnc=0.0.0.0:0 \
-usbdevice tablet
```
## Emulated/Virtualized environment
- Operating system: Windows 11 24H2 / Windows 11 25H2
- OS/kernel version: <!-- For POSIX guests, use `uname -a`. -->
- Architecture: x86_64
## Description of problem
This is kind of adjacent to qemu-project/qemu#3154
When Windows is 'idle', Task Manager showing 0-3% CPU usage, my host system is showing 30% CPU usage from the qemu process in top.
perf kvm --host top showed "read_hpet" as one of the most common calls with an overhead of 10%
I have vaguely been following https://forum.proxmox.com/threads/high-vm-exit-and-host-cpu-usage-on-idle-with-windows-server-2025.163564/page-3#post-819787 but I haven't started *removing* CPU capabilities yet.
Setting -M q35,hpet=off makes the guest unuseably slow.
## Steps to reproduce
1. Install Windows 11 24H2 or 25H2
2. Observe "idle" CPU usage.
3. You can actually see this as soon as the installer starts but I'm not sure this is a fair test.
## Additional information
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Questions:
Are the flags under https://wiki.archlinux.org/title/QEMU#Improve_virtual_machine_performance still correct?
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issue
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