High CPU use on host by idle Windows 7 guest

High CPU use on host by idle Windows 7 guest

[Guido Winkelmann]

Hi,

I have gotten Windows 7 to install and boot on KVM now, but now I'm having 
another problem: The qemu process that the Windows guest is running in is 
constantly using about 27% of one cpu core when idle (more when not, 
obviously).

This is a fresh install of Windows 7 Professional with SP1 done in the same 
qemu virtual machine that it is currently running in, with virtio-scsi for 
disk connectivity and virtio-net for network, using the signed guest drivers 
found here:

http://www.linux-kvm.org/page/Windows7Install

There is nothing else installed on this Windows installation.

(I have seen the same problem with another win7 vm with sata and e1000, 
though.)

After some googling, I found a hint to disable the hpet timer to reduce the 
problem. This didn't work for me, though, the idle cpu use rose to over 60% 
instead.
Another post suggested disabling acpi and/or apic, but that rendered Windows 
unable to boot up.

This is happening with qemu 1.7 and linux kernel 3.12.6 using a SandyBridge 
host cpu.

Qemu has been started with libvirt, the commandline looks like this:

LC_ALL=C 
PATH=/bin:/sbin:/bin:/sbin:/usr/bin:/usr/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin 
HOME=/ USER=root QEMU_AUDIO_DRV=none /usr/bin/qemu-kvm -name 
win7_master2,process=qemu:win7_master2 -S -machine pc-
i440fx-1.7,accel=kvm,usb=off -cpu SandyBridge,+pdpe1gb,+osxsave,+dca,+pcid,
+pdcm,+xtpr,+tm2,+est,+vmx,+ds_cpl,+monitor,+dtes64,+pbe,+tm,+ht,+ss,+acpi,
+ds,+vme -m 4096 -realtime mlock=off -smp 4,sockets=4,cores=1,threads=1 -uuid 
a10e5a00-b1a4-e94d-c5a9-c3079834b400 -no-user-config -nodefaults -chardev 
socket,id=charmonitor,path=/var/lib/libvirt/qemu/win7_master2.monitor,server,nowait 
-mon chardev=charmonitor,id=monitor,mode=control -rtc base=localtime -no-hpet 
-no-shutdown -device piix3-usb-uhci,id=usb,bus=pci.0,addr=0x1.0x2 -device 
virtio-serial-pci,id=virtio-serial0,bus=pci.0,addr=0x5 -drive 
file=/virtual_disks/win7_master2.img,if=none,id=drive-virtio-
disk0,format=qcow2,cache=none -device virtio-blk-
pci,scsi=off,bus=pci.0,addr=0x6,drive=drive-virtio-disk0,id=virtio-
disk0,bootindex=1 -drive file=/virtual_disks/virtio-
win-0.1-74.iso,if=none,id=drive-ide0-0-0,readonly=on,format=raw,cache=none -
device ide-cd,bus=ide.0,unit=0,drive=drive-ide0-0-0,id=ide0-0-0 -drive 
file=/virtual_disks/Win7STICK1_c.iso,if=none,id=drive-
ide0-1-0,readonly=on,format=raw -device ide-cd,bus=ide.1,unit=0,drive=drive-
ide0-1-0,id=ide0-1-0 -netdev tap,fd=21,id=hostnet0 -device virtio-net-
pci,netdev=hostnet0,id=net0,mac=52:54:00:e1:31:22,bus=pci.0,addr=0x3 -chardev 
pty,id=charserial0 -device isa-serial,chardev=charserial0,id=serial0 -device 
usb-tablet,id=input0 -vnc 127.0.0.1:0 -device qxl-
vga,id=video0,ram_size=67108864,vram_size=67108864,bus=pci.0,addr=0x2 -device 
intel-hda,id=sound0,bus=pci.0,addr=0x4 -device hda-duplex,id=sound0-
codec0,bus=sound0.0,cad=0 -device virtio-balloon-
pci,id=balloon0,bus=pci.0,addr=0x7

Does anybody have an idea what's going wrong here? A linux guest (CentOS 6) on 
the same host will not use more than 1% cpu when idle.

Regards,

	Guido