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

[Eduardo Habkost]

On Wed, Aug 23, 2017 at 11:16:27PM +0300, Marcel Apfelbaum wrote:
> 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?

The work has two parts: 1) actually preventing invalid
configurations; 2) providing a query interface to let libvirt
know which configurations are valid.

My focus is on (2) (adding a query-device-slots interface), but
we surely need to address bugs related to (1) first, so both the
device-plugging code and slot-querying code are correct and agree
with each other.

-- 
Eduardo