In libvirt, I found pcie-expander-bus controller doesn't support pcie-to-pci-bridge and pcie-switch-upstream-port.

Version: libvirt-4.9

# cat /tmp/c.xml

...

    <controller type='pci' index='0' model='pcie-root'/>
    <controller type='pci' index='1' model='pcie-expander-bus'>
      <model name='pxb-pcie'/>
      <target busNr='3'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </controller>
    <controller type='pci' index='2' model='pcie-to-pci-bridge'>
      <address type='pci' domain='0x0000' bus='0x01' slot='0x00' function='0x0'/>
    </controller>

...

# virsh -k0 -K0 define /tmp/c.xml                                                                                                                                       
error: Failed to define domain from /tmp/c.xml
error: XML error: The device at PCI address 0000:01:00.0 cannot be plugged into the PCI controller with index='1'. It requires a controller that accepts a pcie-to-pci-bridge.

# cat /tmp/b.xml                
...
    <controller type='pci' index='0' model='pcie-root'/>
    <controller type='pci' index='1' model='pcie-expander-bus'>
      <model name='pxb-pcie'/>
      <target busNr='3'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </controller>
    <controller type='pci' index='2' model='pcie-switch-upstream-port'>
      <model name='x3130-upstream'/>
      <address type='pci' domain='0x0000' bus='0x01' slot='0x00' function='0x0'/>
    </controller>
...

# virsh -k0 -K0 define /tmp/b.xml
error: Failed to define domain from /tmp/b.xml
error: XML error: The device at PCI address 0000:01:00.0 cannot be plugged into the PCI controller with index='1'. It requires a controller that accepts a pci-switch-upstream-port.

In function virDomainPCIAddressBusSetModel, I find pcie-expander-bus only supports pcie-root-port and dmi-to-pci-bridge

   353     case VIR_DOMAIN_CONTROLLER_MODEL_PCIE_EXPANDER_BUS:                        
   354     ┆   /* 32 slots, no hotplug, only accepts pcie-root-port or                
   355     ┆   ┆* dmi-to-pci-bridge                                                   
   356     ┆   ┆*/                                                                    
   357     ┆   bus->flags = (VIR_PCI_CONNECT_TYPE_PCIE_ROOT_PORT |                    
   358     ┆   ┆   ┆   ┆   ┆ VIR_PCI_CONNECT_TYPE_DMI_TO_PCI_BRIDGE);                 
   359     ┆   bus->minSlot = 0;                                                      
   360     ┆   bus->maxSlot = VIR_PCI_ADDRESS_SLOT_LAST;                              
   361     ┆   break;

But it works in qemu:

# /usr/libexec/qemu-kvm -machine q35 -m 1024 -device pxb-pcie,bus_nr=250,id=pci.1,bus=pcie.0,addr=0x4 -device pcie-pci-bridge,id=pci.250,bus=pci.1,addr=0x0 -device pcie-pci-bridge,id=pci.251,bus=pci.1,addr=0x1 -spice port=5902,addr=0.0.0.0,disable-ticketing /var/lib/libvirt/images/q35.qcow2

So why pcie-expander-bus doesn't these two controllers? For any concerns in libvirt?

Thank you~