qemu-system-i386 -hda longhorn.qcow2 -cdrom longhorn.iso -boot d -accel kvm -cpu host -m 1G -usbdevice tablet -vga cirrus -rtc base="2002-09-23",clock=vm
In QEMU/KVM or Proxmox, change the default machine type from q35 to the older (Standard PC). The q35 chipset forces a modern PCIe topology that Longhorn cannot parse, whereas i440fx emulates a classic PCI/IDE layout native to the Windows XP era. CPU Topology windows longhorn qcow2 work