Still trying to get GPU passthrough working and decided to change machine type from older 440fx to q35 ie pc-q35 but if I do virsh edit on the config it refuse to save saying "PCI Bridges not supported"... by current qemu binary. What do I need to fix/change in the virtual machine xml file?

After some research seems q35 only allows PCIexpress PCIe and not PCI but I don't know what to change in my xml file so that virsh stops complaining and accepts change from 440fx to q35.

This is on Debian Jessie: uname -a Linux vmserver 3.16.0-customvmkernel #1 SMP Sun Sep 28 00:34:42 PDT 2014 x86_64 GNU/Linux

kvm --version QEMU emulator version 2.1.0 (Debian 2.1+dfsg-4)

Here is my current virtual machine file with 440fx that I want to change to q35:

<domain type='kvm' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>
<memory unit='KiB'>1048576</memory>
<currentMemory unit='KiB'>1048576</currentMemory>
<vcpu placement='static'>1</vcpu>
<type arch='i686' machine='pc-i440fx-2.1'>hvm</type>
<boot dev='hd'/>
<clock offset='localtime'/>
<disk type='file' device='disk'>
  <driver name='qemu' type='vmdk'/>
  <source file='/mount/ssd/VirtWin7-clone.vmdk'/>
  <target dev='vdb' bus='virtio'/>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x08' function='0x0'/>
<controller type='usb' index='0'>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x2'/>
<controller type='ide' index='0'>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x1'/>
<controller type='pci' index='0' model='pci-root'/>
<interface type='bridge'>
  <mac address='52:54:00:9e:f5:11'/>
  <source bridge='br0'/>
  <model type='rtl8139'/>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
<serial type='pty'>
  <target port='0'/>
<console type='pty'>
  <target type='serial' port='0'/>
<input type='tablet' bus='usb'/>
<input type='mouse' bus='ps2'/>
<input type='keyboard' bus='ps2'/>
<graphics type='vnc' port='-1' autoport='yes'/>
<sound model='ac97'>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
  <model type='cirrus' vram='9216' heads='1'/>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
<memballoon model='virtio'>
<address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
<qemu:arg value='-nodefaults'/>
<qemu:arg value='-vga'/>
<qemu:arg value='none'/>
<qemu:arg value='-device'/>
<qemu:arg value='ioh3420,bus=pci.0,addr=1c.0,multifunction=on,port=1,chassis=1,id=root.1'/>
<qemu:arg value='-device'/>
<qemu:arg value='vfio-pci,host=04:00.0,bus=root.1,addr=00.0,multifunction=on,x-vga=on,rombar=0'/>
<qemu:arg value='-device'/>
<qemu:arg value='vfio-pci,host=04:00.1,bus=root.1,addr=00.1'/>
<qemu:arg value='-bios'/>
<qemu:arg value='/usr/share/seabios/bios.bin'/>
<qemu:arg value='-cpu'/>
<qemu:arg value='host,kvm=off'/>

5 Answers 5


I just did this today - pretty much as 'Fox' described, except change the pci controller entry instead of deleting it - changing 'pci-root' to 'pcie-root'

Then it will complain about anything with a bus-related parameter - I think you can leave the item but remove the specification. I also had to remove USB-related entries (piix3 integrated controller requires a PCI bus, or something to that effect).

Not sure if you're still trying to do this - but I tought I'd put it out there for anyone else who searches for this.

  • 1
    Thanks, I got this resolved some time ago, main thing is I think I was missing the dmi-to-pci bridge and I had to change the buses from 0x00 to 0x02 for some devices, I ended up passing through HDMI+Audio of gpu and also extra USB ports so that mouse/keyboard can work on passed through gpu.
    – htfree
    Apr 16, 2015 at 22:56
  • How do you find how to correct the buses?
    – NoX
    Feb 1, 2019 at 12:21

There is a patch available for the underlying issue (being that QEMU won't allow you to bind to a PCIe root bus, which is needed for Windows guests on Q35):



Latest virt-manager (1.1.0) supports setting architecture on VM creation. You can not change it later.

With virsh edit, I am guessing a little, but I would try removing the PCI controller and all bus related parameters (like <address />. And changing

<type arch='i686' machine='pc-i440fx-2.1'>hvm</type>


<type arch='i686' machine='pc-q35'>hvm</type>

Normally i'd offer this in a comment, since its more of a guess, but it's too long. I'll remove it if it does not work.

btw. those <qemu:arg /> at the end might be causing trouble as well. If you run into problems, try removing those as well.

  • yea, see my comment to Pat, I fixed everything some time ago, with virsh edit ...
    – htfree
    Apr 16, 2015 at 23:03

Try instead of pc-i440fx-2.1 above

See an example at https://gist.github.com/lyarbean/7036986

I'd run qemu manually with -M q35 or -M ? to check the version you have installed really supports it


  • sorry just noticed reply, if I change to pc-q35 I get "error: unsupported configuration: PCI bridges are not supported by this QEMU binary"
    – htfree
    Nov 2, 2014 at 7:32
  • got this resolved some time ago, change pci to pcie etc and fixed bus addresses...
    – htfree
    Apr 16, 2015 at 23:01

First you need to check if qemu-kvm supports q35, qemu 2.x supports Q35 , you can check if supported machine using

 1.   [root@localhost ~]# /usr/libexec/qemu-kvm -machine help
    Supported machines are:
    pc                   RHEL 7.6.0 PC (i440FX + PIIX, 1996) (alias of pc-i440fx-rhel7.6.0)
    pc-i440fx-rhel7.6.0  RHEL 7.6.0 PC (i440FX + PIIX, 1996) (default)
    pc-i440fx-rhel7.5.0  RHEL 7.5.0 PC (i440FX + PIIX, 1996)
    pc-i440fx-rhel7.4.0  RHEL 7.4.0 PC (i440FX + PIIX, 1996)
    pc-i440fx-rhel7.3.0  RHEL 7.3.0 PC (i440FX + PIIX, 1996)
    pc-i440fx-rhel7.2.0  RHEL 7.2.0 PC (i440FX + PIIX, 1996)
    rhel6.1.0            RHEL 6.1.0 PC
    rhel6.0.0            RHEL 6.0.0 PC
    q35                  RHEL-7.6.0 PC (Q35 + ICH9, 2009) (alias of pc-q35-rhel7.6.0)
    pc-q35-rhel7.6.0     RHEL-7.6.0 PC (Q35 + ICH9, 2009)
    pc-q35-rhel7.5.0     RHEL-7.5.0 PC (Q35 + ICH9, 2009)
    pc-q35-rhel7.4.0     RHEL-7.4.0 PC (Q35 + ICH9, 2009)
    pc-q35-rhel7.3.0     RHEL-7.3.0 PC (Q35 + ICH9, 2009)
    none                 empty machine

#use virsh edit to change machine type from above list


if using commnad line

  /usr/bin/qemu-system-x86_64  -machine q35,kernel-irqchip=split,accel=kvm -smp 4 -m 2G -device intel-iommu,intremap=on,caching-mode=on -nographic /home/fastlinq/centos-7.8.qcow2
  • qemu-system-x86_64 --machine help worked for me. Thanks!
    – user643011
    Sep 14, 2020 at 6:11

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