I just noticed a patch recently submitted to qemu, attempting to lower the cpu load for idle vm using usb tablet emulation.
This is the comment of commit 427e3aa151c749225364d0c30640e2e3c1756d9d:
usb-tablet: Allow connecting to ehci
Our ehci code has is capable of significantly lowering the wakeup rate
for the hcd emulation while the device is idle. It is possible to add
similar code ot the uhci emulation, but that simply is not there atm,
and there is no reason why a (virtual) usb-tablet can not be a USB-2 device.
Making usb-hid devices connect to the emulated ehci controller instead
of the emulated uhci controller on vms which have both lowers the cpuload
for a fully idle vm from 20% to 2-3% (on my laptop).
An alternative implementation to using a property to select the tablet
type, would be simply making it a new device type, ie usb-tablet2, but the
downside of that is that this will require libvirt changes to be available
through libvirt at all, and then management tools changes to become the
default for new vms, where as using a property will automatically get
any pc-1.3 type vms the lower cpuload.
I tried to connect usb tablet to ehci by libvirt, which automatically generted qemu command as follows. However, the cpuload of idle vm is still much higher than PS2 mouse emulation on my laptop. (usb-tablet: 30-35%, usb-tablet2: 16-19%, PS2 mouse: 9-10%). Is there something wrong with my configuration?
57 <controller type='usb' index='0' model='ehci'/>
58 <input type='tablet' bus='usb'>
59 <address type='usb' bus='0' port='1'/>
60 </input>
/usr/bin/qemu-kvm -name winxp -S -cpu host -enable-kvm -m 1024 -smp 2,sockets=2,cores=1,threads=1 -uuid 883d614a-30e9-474d-b81e-c131e4a06249 -no-user-config -nodefaults -chardev socket,id=charmonitor,path=/var/lib/libvirt/qemu/winxp.monitor,server,nowait -mon chardev=charmonitor,id=monitor,mode=control -rtc base=localtime -no-shutdown -device usb-ehci,id=usb,bus=pci.0,addr=0x5 -device virtio-serial-pci,id=virtio-serial0,bus=pci.0,addr=0x6 -drive file=/home/vm/winxp/winxp.img,if=none,id=drive-virtio-disk0,format=qcow2,cache=none,aio=native -device virtio-blk-pci,scsi=off,bus=pci.0,addr=0x7,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1 -netdev tap,fd=23,id=hostnet0,vhost=on,vhostfd=24 -device virtio-net-pci,netdev=hostnet0,id=net0,mac=52:54:00:65:f3:de,bus=pci.0,addr=0x3 -chardev spicevmc,id=charchannel0,name=vdagent -device virtserialport,bus=virtio-serial0.0,nr=1,chardev=charchannel0,id=channel0,name=com.redhat.spice.0 -device usb-tablet,id=input0,bus=usb.0,port=1 -spice port=5900,addr=0.0.0.0,disable-ticketing,seamless-migration=on -vga qxl -global qxl-vga.vram_size=67108864 -device AC97,id=sound0,bus=pci.0,addr=0x4