qemu/docs/pcie_pci_bridge.txt

16 can be hot-plugged into appropriate root port (requires additional actions,
29 any device plugged in, has no free buses reserved to provide any of them
30 to a hot-plugged devices in future.
36 that is planned to have PCIE-PCI bridge hot-plugged in.
86  (1) Cold-plugged:
89     - 2 PCIE-PCI bridges plugged into 2 different root ports;
90     - e1000 plugged into the first bridge.
91  (2) Hot-plugged:
92     - PCIE-PCI bridge, plugged into QEMU generic root port;
93     - 2 e1000 cards, one plugged into the cold-plugged PCIE-PCI bridge,
94                      another plugged into the hot-plugged bridge.
98 The PCIE-PCI bridge can be hot-plugged only into pcie-root-port that
100 hot-plugged bridge.