[Qemu-devel] Number of usable slots in PCIe Root Port / PCIe Switch Downstream Port

[Andrea Bolognani]

AFAIK some PCI controllers, particularly:

  * PCIe Root Port (pcie-root-port, ioh3420)
  * PCIe Switch Downstream Port (xio3130-downstream)

only have a single usable slot. libvirt knows about this
fact, and will prevent you from adding more than one
device to the respective bus.

However, as Lukáš recently noticed, QEMU won't complain
if you add more devices:

  $ qemu-system-x86_64 \
    -nodefaults -nographic \
    -M q35 -monitor stdio \
    -device pcie-root-port,id=pci.1 \
    -device virtio-scsi-pci,bus=pci.1 \
    -device virtio-scsi-pci,bus=pci.1
  QEMU 2.9.0 monitor - type 'help' for more information
  (qemu) info qtree
  bus: main-system-bus
    type System
    [...]
    dev: q35-pcihost, id ""
      [...]
      bus: pcie.0
        type PCIE
        dev: pcie-root-port, id "pci.1"
          [...]
          bus: pci.1
            type PCIE
            dev: virtio-scsi-pci, id ""
              [...]
              addr = 01.0
              bus: virtio-bus
                type virtio-pci-bus
                dev: virtio-scsi-device, id ""
                  [...]
                  bus: scsi.1
                    type SCSI
            dev: virtio-scsi-pci, id ""
              [...]
              addr = 00.0
              bus: virtio-bus
                type virtio-pci-bus
                dev: virtio-scsi-device, id ""
                  [...]
                  bus: scsi.0
                    type SCSI
  (qemu)

As you can see, all devices will show up in the qtree;
only the one with addr=00.0, however, will actually be
visible to the guest OS according to my tests.

Is such a configuration considered valid? Should QEMU
complain loudly about it and refuse to start? Or should
libvirt and the guest OS / firmware start allowing it?

-- 
Andrea Bolognani / Red Hat / Virtualization