The nec-usb-xhci device (which is a USB3 controller) has always
presented itself as a PCI device when plugged into a legacy PCI slot,
and a PCIe device when plugged into a PCIe slot, but libvirt has
always auto-assigned it to a legacy PCI slot.
This patch changes that behavior to auto-assign to a PCIe slot on
systems that have pcie-root (e.g. Q35 and aarch64/virt).
Since we don't yet auto-create pcie-*-port controllers on demand, this
means a config with an nec-xhci USB controller that has no PCI address
assigned will also need to have an otherwise-unused pcie-*-port
controller specified:
<controller type='pci' model='pcie-root-port'/>
<controller type='usb' model='nec-xhci'/>
(this assumes there is an otherwise-unused slot on pcie-root to accept
the pcie-root-port)
Hand-entering indexes for 20 PCI controllers is not as tedious as
manually determining and entering their PCI addresses, but it's still
annoying, and the algorithm for determining the proper index is
incredibly simple (in all cases except one) - just pick the lowest
unused index.
The one exception is USB2 controllers because multiple controllers in
the same group have the same index. For these we look to see if 1) the
most recently added USB controller is also a USB2 controller, and 2)
the group *that* controller belongs to doesn't yet have a controller
of the exact model we're just now adding - if both are true, the new
controller gets the same index, but in all other cases we just assign
the lowest unused index.
With this patch in place and combined with the automatic PCI address
assignment, we can define a PCIe switch with several ports like this:
<controller type='pci' model='pcie-root-port'/>
<controller type='pci' model='pcie-switch-upstream-port'/>
<controller type='pci' model='pcie-switch-downstream-port'/>
<controller type='pci' model='pcie-switch-downstream-port'/>
<controller type='pci' model='pcie-switch-downstream-port'/>
<controller type='pci' model='pcie-switch-downstream-port'/>
<controller type='pci' model='pcie-switch-downstream-port'/>
...
These will each get a unique index, and PCI addresses that connect
them together appropriately with no pesky numbers required.
