Re: [libvirt] [PATCH v6 11/17] [RFC] qemu: if pci-bridge is in PCIe config w/o dmi-to-pci-bridge, add it

I'm undecided if it is worthwhile to add this...   Up until now it has been legal to have something like this in the xml:      <controller type='pci' model='pcie-root' index='0'/>    <controller type='pci' model='pci-bridge' index='2'/>   (for example, see the existing test case "usb-controller-default-q35").  This is handled in qemuDomainPCIAddressSetCreate() when it's adding in controllers to fill holes in the indexes.)   Since we have always added the following to every config:      <controller type='pci' model='dmi-to-pci-bridge' index='1'/>   there has always been a proper place for the unaddressed pci-bridge to plug in.   A upcoming commit will eliminate the forced adding of a dmi-to-pci-bridge to *every* Q35 domain config, though. This would mean the above-mentioned test case (and one other) would begin to fail. There are two possible solutions to this problem:   1) change the test cases, and made the above construct an error (note that in the future it will work just fine to not list *any* pci controller, and they will all be auto-added as needed).   or   2) This patch, which specifically looks for the case where we have a pci-bridge (but no dmi-to-pci-bridge) in a PCIe domain config and auto-adds the necessary dmi-to-pci-bridge. This can only work in the case that the pci-bridge has been given an index such that there is an unused index with a lower value for the dmi-to-pci-bridge to occupy.   This seems like a kind of odd corner case that nobody should need to do in the future anyway. On the other hand, I already wrote the code and it works, so I might as well send it and see what opinions (if any) it generates.