qemu-system-x86 process at ~30% CPU even when Linux VM is 99-100% idle

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I'm running OpenWRT inside a VM that is only used for WireGuard VPN as my router doesn't support it itself. The VM is running at 99-100% idle, according to top inside the VM. The qemu-system-x86 process on the Unraid host on the other hand runs at a constant ~30% CPU usage according to top. Note: I'm on Unraid 6.9.0-rc.2, the system itself is a repurposed QNAP TS-451A with a slow Celeron N3060, so having 30% of the CPU blocked full-time is quite annoying.


top in the VM:


Mem: 40100K used, 203720K free, 392K shrd, 9608K buff, 8828K cached
CPU:   0% usr   0% sys   0% nic  99% idle   0% io   0% irq   0% sirq
Load average: 0.00 0.04 0.01 2/64 2088
 2071  1430 root     S     1000   0%   0% /usr/sbin/dropbear -F -P /var/run/dropbear.1.pid -p 22 -K 300 -T 3
 1801     1 root     SN    3484   1%   0% /usr/sbin/collectd -C /tmp/collectd.conf -f
 1591     1 root     S     1384   1%   0% /usr/sbin/uhttpd -f -h /www -r OpenWrt -x /cgi-bin -t 60 -T 30 -k 20 -A 1 -n 3 -N 100 -R -p
 1334     1 root     S     1320   1%   0% /sbin/rpcd -s /var/run/ubus.sock -t 30
 1485     1 root     S     1268   1%   0% /sbin/netifd
    1     0 root     S     1160   0%   0% /sbin/procd
 1397     1 dnsmasq  S     1120   0%   0% /usr/sbin/dnsmasq -C /var/etc/dnsmasq.conf.cfg01411c -k -x /var/run/dnsmasq/dnsmasq.cfg01411c.pid
 2072  2071 root     S     1084   0%   0% -ash
 2080  2072 root     R     1080   0%   0% top
 1968     1 root     S<    1076   0%   0% /usr/sbin/ntpd -n -N -S /usr/sbin/ntpd-hotplug -p 0.openwrt.pool.ntp.org -p 1.openwrt.pool.ntp.org -p 2
 1606  1485 root     S     1072   0%   0% udhcpc -p /var/run/udhcpc-eth0.pid -s /lib/netifd/dhcp.script -f -t 0 -i eth0 -x hostname:OpenWrt -C -O
 1519     1 root     S     1064   0%   0% /usr/sbin/odhcpd
 1303     1 root     S      964   0%   0% /sbin/logd -S 64
 1131     1 root     S      888   0%   0% /sbin/ubusd
 1430     1 root     S      888   0%   0% /usr/sbin/dropbear -F -P /var/run/dropbear.1.pid -p 22 -K 300 -T 3
 1146     1 root     S      780   0%   0% /sbin/urngd
 1133     1 root     S      720   0%   0% /sbin/askfirst /usr/libexec/login.sh
 1132     1 root     S      720   0%   0% /sbin/askfirst /usr/libexec/login.sh


top on the Unraid host:


top - 22:45:54 up 1 day,  7:22,  1 user,  load average: 0.27, 0.47, 0.52
Tasks: 204 total,   1 running, 203 sleeping,   0 stopped,   0 zombie
%Cpu(s):  3.7 us, 15.4 sy,  0.0 ni, 80.7 id,  0.2 wa,  0.0 hi,  0.0 si,  0.0 st
MiB Mem :   7879.5 total,    184.3 free,    421.6 used,   7273.6 buff/cache
MiB Swap:      0.0 total,      0.0 free,      0.0 used.   6412.5 avail Mem

29335 root      20   0  620200 181356  22816 S  28.9   2.2   1:55.99 qemu-system-x86


I tried assigning only one or both of the CPU threads, removing all USB devices and switching the machine type from Q35 to i440fx, nothing helped with the idle CPU usage.


VM configuration:


<?xml version='1.0' encoding='UTF-8'?>
<domain type='kvm' id='6'>
    <vmtemplate xmlns="unraid" name="Linux" icon="linux.png" os="linux"/>
  <memory unit='KiB'>262144</memory>
  <currentMemory unit='KiB'>262144</currentMemory>
  <vcpu placement='static'>1</vcpu>
    <vcpupin vcpu='0' cpuset='0'/>
    <type arch='x86_64' machine='pc-i440fx-5.1'>hvm</type>
  <cpu mode='host-passthrough' check='none' migratable='on'>
    <topology sockets='1' dies='1' cores='1' threads='1'/>
    <cache mode='passthrough'/>
  <clock offset='utc'>
    <timer name='rtc' tickpolicy='catchup'/>
    <timer name='pit' tickpolicy='delay'/>
    <timer name='hpet' present='no'/>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2' cache='writeback'/>
      <source file='/mnt/cache-sdcard/domains/openwrt/vdisk1.qcow2' index='1'/>
      <target dev='hdc' bus='virtio'/>
      <boot order='1'/>
      <alias name='virtio-disk2'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
    <controller type='usb' index='0' model='ich9-ehci1'>
      <alias name='usb'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x07' function='0x7'/>
    <controller type='usb' index='0' model='ich9-uhci1'>
      <alias name='usb'/>
      <master startport='0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x07' function='0x0' multifunction='on'/>
    <controller type='usb' index='0' model='ich9-uhci2'>
      <alias name='usb'/>
      <master startport='2'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x07' function='0x1'/>
    <controller type='usb' index='0' model='ich9-uhci3'>
      <alias name='usb'/>
      <master startport='4'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x07' function='0x2'/>
    <controller type='pci' index='0' model='pci-root'>
      <alias name='pci.0'/>
    <controller type='virtio-serial' index='0'>
      <alias name='virtio-serial0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
    <interface type='bridge'>
      <mac address='52:54:00:76:94:0f'/>
      <source bridge='br0'/>
      <target dev='vnet0'/>
      <model type='virtio-net'/>
      <alias name='net0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    <serial type='pty'>
      <source path='/dev/pts/0'/>
      <target type='isa-serial' port='0'>
        <model name='isa-serial'/>
      <alias name='serial0'/>
    <console type='pty' tty='/dev/pts/0'>
      <source path='/dev/pts/0'/>
      <target type='serial' port='0'/>
      <alias name='serial0'/>
    <channel type='unix'>
      <source mode='bind' path='/var/lib/libvirt/qemu/channel/target/domain-6-OpenWRT/org.qemu.guest_agent.0'/>
      <target type='virtio' name='org.qemu.guest_agent.0' state='disconnected'/>
      <alias name='channel0'/>
      <address type='virtio-serial' controller='0' bus='0' port='1'/>
    <input type='tablet' bus='usb'>
      <alias name='input0'/>
      <address type='usb' bus='0' port='1'/>
    <input type='mouse' bus='ps2'>
      <alias name='input1'/>
    <input type='keyboard' bus='ps2'>
      <alias name='input2'/>
    <graphics type='vnc' port='5900' autoport='yes' websocket='5700' listen='' keymap='en-us'>
      <listen type='address' address=''/>
      <model type='qxl' ram='65536' vram='65536' vgamem='16384' heads='1' primary='yes'/>
      <alias name='video0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
    <memballoon model='virtio'>
      <alias name='balloon0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x06' function='0x0'/>
  <seclabel type='dynamic' model='dac' relabel='yes'>


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I played around with this a bit more and changing <timer name='hpet' present='no'/> to yes reduced the idle CPU usage down to ~5% on the Unraid host, which is much more acceptable. I'll have to monitor the VM since I'm not really sure if this has any downsides.

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