pcie_pci_bridge.txt (4538B)
1 Generic PCI Express to PCI Bridge 2 ================================ 3 4 Description 5 =========== 6 PCIE-to-PCI bridge is a new method for legacy PCI 7 hierarchies creation on Q35 machines. 8 9 Previously Intel DMI-to-PCI bridge was used for this purpose. 10 But due to its strict limitations - no support of hot-plug, 11 no cross-platform and cross-architecture support - a new generic 12 PCIE-to-PCI bridge should now be used for any legacy PCI device usage 13 with PCI Express machine. 14 15 This generic PCIE-PCI bridge is a cross-platform device, 16 can be hot-plugged into appropriate root port (requires additional actions, 17 see 'PCIE-PCI bridge hot-plug' section), 18 and supports devices hot-plug into the bridge itself 19 (with some limitations, see below). 20 21 Hot-plug of legacy PCI devices into the bridge 22 is provided by bridge's built-in Standard hot-plug Controller. 23 Though it still has some limitations, see below. 24 25 PCIE-PCI bridge hot-plug 26 ======================= 27 Guest OSes require extra efforts to enable PCIE-PCI bridge hot-plug. 28 Motivation - now on init any PCI Express root port which doesn't have 29 any device plugged in, has no free buses reserved to provide any of them 30 to a hot-plugged devices in future. 31 32 To solve this problem we reserve additional buses on a firmware level. 33 Currently only SeaBIOS is supported. 34 The way of bus number to reserve delivery is special 35 Red Hat vendor-specific PCI capability, added to the root port 36 that is planned to have PCIE-PCI bridge hot-plugged in. 37 38 Capability 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 59 If any reservation field is -1 then this kind of reservation is not 60 needed and must be ignored by firmware. 61 62 At the moment this capability is used only in QEMU generic PCIe root port 63 (-device pcie-root-port). Capability construction function takes all reservation 64 fields values from corresponding device properties. By default all of them are 65 set to -1 to leave root port's default behavior unchanged. 66 67 Usage 68 ===== 69 A detailed command line would be: 70 71 [qemu-bin + storage options] \ 72 -m 2G \ 73 -device pcie-root-port,bus=pcie.0,id=rp1,slot=1 \ 74 -device pcie-root-port,bus=pcie.0,id=rp2,slot=2 \ 75 -device pcie-root-port,bus=pcie.0,id=rp3,slot=3,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 80 Then in monitor it's OK to execute next commands: 81 device_add pcie-pci-bridge,id=br3,bus=rp3 \ 82 device_add e1000,bus=br2,addr=1 \ 83 device_add e1000,bus=br3,addr=1 84 85 Here 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 96 Limitations 97 =========== 98 The PCIE-PCI bridge can be hot-plugged only into pcie-root-port that 99 has proper 'bus-reserve' property value to provide secondary bus for the 100 hot-plugged bridge. 101 102 Windows 7 and older versions don't support hot-plug devices into the PCIE-PCI bridge. 103 To enable device hot-plug into the bridge on Linux there're 3 ways: 104 1) Build shpchp module with this patch http://www.spinics.net/lists/linux-pci/msg63052.html 105 2) Use kernel 4.14+ where the patch mentioned above is already merged. 106 3) 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. 109 110 Implementation 111 ============== 112 The PCIE-PCI bridge is based on PCI-PCI bridge, but also accumulates PCI Express 113 features as a PCI Express device. 114