993e38020f
This has been the default forever. Acked-by: Alex Bennée <alex.bennee@linaro.org> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
59 lines
2.7 KiB
Plaintext
59 lines
2.7 KiB
Plaintext
PCI EXPANDER BRIDGE (PXB)
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=========================
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Description
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===========
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PXB is a "light-weight" host bridge in the same PCI domain
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as the main host bridge whose purpose is to enable
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the main host bridge to support multiple PCI root buses.
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It is implemented only for i440fx and can be placed only
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on bus 0 (pci.0).
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As opposed to PCI-2-PCI bridge's secondary bus, PXB's bus
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is a primary bus and can be associated with a NUMA node
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(different from the main host bridge) allowing the guest OS
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to recognize the proximity of a pass-through device to
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other resources as RAM and CPUs.
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Usage
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=====
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A detailed command line would be:
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[qemu-bin + storage options]
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-m 2G
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-object memory-backend-ram,size=1024M,policy=bind,host-nodes=0,id=ram-node0 -numa node,nodeid=0,cpus=0,memdev=ram-node0
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-object memory-backend-ram,size=1024M,policy=bind,host-nodes=1,id=ram-node1 -numa node,nodeid=1,cpus=1,memdev=ram-node1
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-device pxb,id=bridge1,bus=pci.0,numa_node=1,bus_nr=4 -netdev user,id=nd -device e1000,bus=bridge1,addr=0x4,netdev=nd
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-device pxb,id=bridge2,bus=pci.0,numa_node=0,bus_nr=8 -device e1000,bus=bridge2,addr=0x3
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-device pxb,id=bridge3,bus=pci.0,bus_nr=40 -drive if=none,id=drive0,file=[img] -device virtio-blk-pci,drive=drive0,bus=bridge3,addr=1
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Here you have:
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- 2 NUMA nodes for the guest, 0 and 1. (both mapped to the same NUMA node in host, but you can and should put it in different host NUMA nodes)
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- a pxb host bridge attached to NUMA 1 with an e1000 behind it
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- a pxb host bridge attached to NUMA 0 with an e1000 behind it
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- a pxb host bridge not attached to any NUMA with a hard drive behind it.
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Limitations
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===========
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Please observe that we specified the bus "pci.0" for the second and third pxb.
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This is because when no bus is given, another pxb can be selected by QEMU as default bus,
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however, PXBs can be placed only under the root bus.
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Implementation
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==============
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The PXB is composed by:
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- HostBridge (TYPE_PXB_HOST)
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The host bridge allows to register and query the PXB's PCI root bus in QEMU.
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- PXBDev(TYPE_PXB_DEVICE)
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It is a regular PCI Device that resides on the piix host-bridge bus and its bus uses the same PCI domain.
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However, the bus behind is exposed through ACPI as a primary PCI bus and starts a new PCI hierarchy.
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The interrupts from devices behind the PXB are routed through this device the same as if it were a
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PCI-2-PCI bridge. The _PRT follows the i440fx model.
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- PCIBridgeDev(TYPE_PCI_BRIDGE_DEV)
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Created automatically as part of init sequence.
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When adding a device to PXB it is attached to the bridge for two reasons:
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- Using the bridge will enable hotplug support
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- All the devices behind the bridge will use bridge's IO/MEM windows compacting
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the PCI address space.
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