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

[Marcel Apfelbaum <marcel@redhat.com>]

On 23/08/2017 19:46, Andrea Bolognani wrote:
> 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)
> 

Hi Andrea,

> 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? 

Definitely no.

>Should QEMU
> complain loudly about it and refuse to start? 

Yes, but we need a clean way to do it. Eduardo is working
on a series that would expose to libvirt this info,
Eduardo can you please confirm?

Or should
> libvirt and the guest OS / firmware start allowing it?
> 

I thought libvirt does not allow it anyway and yes, is a bug,
but considered low priority.
I personally hoped Eduardo's work can also help in this direction
(classifying PCI Buses, we can add a 'rule' to allow only one
PCIe device per bus.)

But maybe a simple patch preventing QEMU to start would be enough.

Thanks for bringing the issue to my attention,
Marcel