1Generic PCI Express to PCI Bridge 2================================ 3 4Description 5=========== 6PCIE-to-PCI bridge is a new method for legacy PCI 7hierarchies creation on Q35 machines. 8 9Previously Intel DMI-to-PCI bridge was used for this purpose. 10But due to its strict limitations - no support of hot-plug, 11no cross-platform and cross-architecture support - a new generic 12PCIE-to-PCI bridge should now be used for any legacy PCI device usage 13with PCI Express machine. 14 15This generic PCIE-PCI bridge is a cross-platform device, 16can be hot-plugged into appropriate root port (requires additional actions, 17see 'PCIE-PCI bridge hot-plug' section), 18and supports devices hot-plug into the bridge itself 19(with some limitations, see below). 20 21Hot-plug of legacy PCI devices into the bridge 22is provided by bridge's built-in Standard hot-plug Controller. 23Though it still has some limitations, see below. 24 25PCIE-PCI bridge hot-plug 26======================= 27Guest OSes require extra efforts to enable PCIE-PCI bridge hot-plug. 28Motivation - now on init any PCI Express root port which doesn't have 29any device plugged in, has no free buses reserved to provide any of them 30to a hot-plugged devices in future. 31 32To solve this problem we reserve additional buses on a firmware level. 33Currently only SeaBIOS is supported. 34The way of bus number to reserve delivery is special 35Red Hat vendor-specific PCI capability, added to the root port 36that is planned to have PCIE-PCI bridge hot-plugged in. 37 38Capability layout (defined in include/hw/pci/pci_bridge.h): 39 40 uint8_t id; Standard PCI capability header field 41 uint8_t next; Standard PCI capability header field 42 uint8_t len; Standard PCI vendor-specific capability header field 43 44 uint8_t type; Red Hat vendor-specific capability type 45 List of currently existing types: 46 RESOURCE_RESERVE = 1 47 48 49 uint32_t bus_res; Minimum number of buses to reserve 50 51 uint64_t io; IO space to reserve 52 uint32_t mem Non-prefetchable memory to reserve 53 54 At most one of the following two fields may be set to a value 55 different from -1: 56 uint32_t mem_pref_32; Prefetchable memory to reserve (32-bit MMIO) 57 uint64_t mem_pref_64; Prefetchable memory to reserve (64-bit MMIO) 58 59If any reservation field is -1 then this kind of reservation is not 60needed and must be ignored by firmware. 61 62At the moment this capability is used only in QEMU generic PCIe root port 63(-device pcie-root-port). Capability construction function takes all reservation 64fields values from corresponding device properties. By default all of them are 65set to -1 to leave root port's default behavior unchanged. 66 67Usage 68===== 69A detailed command line would be: 70 71[qemu-bin + storage options] \ 72-m 2G \ 73-device pcie-root-port,bus=pcie.0,id=rp1 \ 74-device pcie-root-port,bus=pcie.0,id=rp2 \ 75-device pcie-root-port,bus=pcie.0,id=rp3,bus-reserve=1 \ 76-device pcie-pci-bridge,id=br1,bus=rp1 \ 77-device pcie-pci-bridge,id=br2,bus=rp2 \ 78-device e1000,bus=br1,addr=8 79 80Then in monitor it's OK to execute next commands: 81device_add pcie-pci-bridge,id=br3,bus=rp3 \ 82device_add e1000,bus=br2,addr=1 \ 83device_add e1000,bus=br3,addr=1 84 85Here you have: 86 (1) Cold-plugged: 87 - Root ports: 1 QEMU generic root port with the capability mentioned above, 88 2 QEMU generic root ports without this capability; 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. 95 96Limitations 97=========== 98The PCIE-PCI bridge can be hot-plugged only into pcie-root-port that 99has proper 'bus-reserve' property value to provide secondary bus for the 100hot-plugged bridge. 101 102Windows 7 and older versions don't support hot-plug devices into the PCIE-PCI bridge. 103To enable device hot-plug into the bridge on Linux there're 3 ways: 1041) Build shpchp module with this patch http://www.spinics.net/lists/linux-pci/msg63052.html 1052) Use kernel 4.14+ where the patch mentioned above is already merged. 1063) Set 'msi' property to off - this forces the bridge to use legacy INTx, 107 which allows the bridge to notify the OS about hot-plug event without having 108 BUSMASTER set. 109 110Implementation 111============== 112The PCIE-PCI bridge is based on PCI-PCI bridge, but also accumulates PCI Express 113features as a PCI Express device. 114 115