+    } else if (flags & VIR_PCI_CONNECT_TYPE_PCI_BRIDGE) {
> +        model = VIR_DOMAIN_CONTROLLER_MODEL_DMI_TO_PCI_BRIDGE;

I'm not so clear on this bit: if we're growing the address
set to plug in a pci-bridge, can we just go ahead and assume
a dmi-to-pci-bridge is what we need? What about eg. pseries
machine types, where dmi-to-pci-bridge is not usable?

What can you plug a pci-bridge into on a pseries machine? For all machinetypes, if there is an unaddressed pci-bridge in the config and there is a slot available that accepts a pci-bridge (i.e. pci-root, pci-bridge, or dmi-to-pci-bridge) then we won't be calling virDomainPCIAddressSetGrow() in the first place (since it's only called if no empty slot with correct flags can be found). And I don't know about pseries, but for 440fx, if there isn't a matching empty slot that accepts a pci-bridge at that point, then it's not possible to add one (you already have the one and only pci-root, and you're already trying to add a pci-bridge --> if you have to add a pci-bridge in order to add a pci-bridge, then you're in an infinite recursion).

So a dmi-to-pci-bridge is the only thing that could possibly help, and in the cases where there is no pcie-root, it would just mean that you would get an error later when you're told there's no place to plug in the dmi-to-pci-bridge. So it makes sense for me to add a check here to see if the model of addrs->buses[0] is PCIE_ROOT, and only try adding the dmi-to-pci-bridge in that case.