为什么libvirt要创建多个PCI？

Question

我在qemu-system-x86_64命令的参数中运行libvirt，发现它创建了多个PCI：

/usr/bin/qemu-system-x86_64 \
    ...
    -device '{"driver":"pcie-root-port","port":16,"chassis":1,"id":"pci.1","bus":"pcie.0","multifunction":true,"addr":"0x2"}' \
    -device '{"driver":"pcie-root-port","port":17,"chassis":2,"id":"pci.2","bus":"pcie.0","addr":"0x2.0x1"}' \
    -device '{"driver":"pcie-root-port","port":18,"chassis":3,"id":"pci.3","bus":"pcie.0","addr":"0x2.0x2"}' \
    -device '{"driver":"pcie-root-port","port":19,"chassis":4,"id":"pci.4","bus":"pcie.0","addr":"0x2.0x3"}' \
    -device '{"driver":"pcie-root-port","port":20,"chassis":5,"id":"pci.5","bus":"pcie.0","addr":"0x2.0x4"}' \
    -device '{"driver":"pcie-root-port","port":21,"chassis":6,"id":"pci.6","bus":"pcie.0","addr":"0x2.0x5"}' \
    -device '{"driver":"pcie-root-port","port":22,"chassis":7,"id":"pci.7","bus":"pcie.0","addr":"0x2.0x6"}' \
    -device '{"driver":"pcie-root-port","port":23,"chassis":8,"id":"pci.8","bus":"pcie.0","addr":"0x2.0x7"}' \
    -device '{"driver":"pcie-root-port","port":24,"chassis":9,"id":"pci.9","bus":"pcie.0","multifunction":true,"addr":"0x3"}' \
    -device '{"driver":"pcie-root-port","port":25,"chassis":10,"id":"pci.10","bus":"pcie.0","addr":"0x3.0x1"}' \
    -device '{"driver":"pcie-root-port","port":26,"chassis":11,"id":"pci.11","bus":"pcie.0","addr":"0x3.0x2"}' \
    -device '{"driver":"pcie-root-port","port":27,"chassis":12,"id":"pci.12","bus":"pcie.0","addr":"0x3.0x3"}' \
    -device '{"driver":"pcie-root-port","port":28,"chassis":13,"id":"pci.13","bus":"pcie.0","addr":"0x3.0x4"}' \
    -device '{"driver":"pcie-root-port","port":29,"chassis":14,"id":"pci.14","bus":"pcie.0","addr":"0x3.0x5"}' \
    -device '{"driver":"pcie-root-port","port":30,"chassis":15,"id":"pci.15","bus":"pcie.0","addr":"0x3.0x6"}' \
    -device '{"driver":"pcie-pci-bridge","id":"pci.16","bus":"pci.6","addr":"0x0"}' \
    ...

在默认情况下，qemu似乎只创建一个PCI。为虚拟IO设备创建多个PCI有什么好处吗？

Answer 1

使用PCI拓扑结构，每个单独的设备(磁盘、网卡、视频、声音等)都需要连接到专用的pcie-root-port上。您不能热插拔/拔出pcie-root-port设备本身，但如果您已经有了pcie-root-port，则可以将设备热插即用/拔出。因此，为了允许您在运行时热插拔磁盘/nics/etc，需要预先创建额外的pcie-root-port设备。Libvirt只会预先创建一个备用程序，但是使用libvirt的应用程序可以要求尽可能多的额外服务。看到有30个端口的配置，我猜您可能一直在使用OpenStack，因为我知道这会预先创建许多额外的pcie-root-port，从而允许很多热插拔选项。