Re: [Qemu-devel] [qemu-s390x] [PATCH v2 2/6] s390x/pci: Fix hotplugging of PCI bridges

[Collin Walling <walling@linux.ibm.com>]

On 1/30/19 10:57 AM, David Hildenbrand wrote:
> When hotplugging a PCI bridge right now to the root port, we resolve
> pci_get_bus(pdev)->parent_dev, which results in a SEGFAULT. Hotplugging
> really only works right now when hotplugging to another bridge.
> 
> Instead, we have to properly check if we are already at the root.
> 
> Let's cleanup the code while at it a bit and factor out updating the
> subordiante bus number into a separate function. The check for

s/subordiante/subordinate

> "old_nr < nr" is right now not strictly necessary, but makes it more
> obvious what is actually going on.
> 
> Most probably fixing up the topology is not our responsibility when
> hotplugging. The guest has to sort this out. But let's keep it for now
> and only fix current code to not crash.
> 
> Reviewed-by: Thomas Huth <thuth@redhat.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>   hw/s390x/s390-pci-bus.c | 28 +++++++++++++++++++---------
>   1 file changed, 19 insertions(+), 9 deletions(-)
> 
> diff --git a/hw/s390x/s390-pci-bus.c b/hw/s390x/s390-pci-bus.c
> index b7c4613fde..9b5c5fff60 100644
> --- a/hw/s390x/s390-pci-bus.c
> +++ b/hw/s390x/s390-pci-bus.c
> @@ -843,6 +843,21 @@ static void s390_pcihost_pre_plug(HotplugHandler *hotplug_dev, DeviceState *dev,
>       }
>   }
>   
> +static void s390_pci_update_subordinate(PCIDevice *dev, uint32_t nr)
> +{
> +    uint32_t old_nr;
> +
> +    pci_default_write_config(dev, PCI_SUBORDINATE_BUS, nr, 1);
> +    while (!pci_bus_is_root(pci_get_bus(dev))) {
> +        dev = pci_get_bus(dev)->parent_dev;
> +
> +        old_nr = pci_default_read_config(dev, PCI_SUBORDINATE_BUS, 1);
> +        if (old_nr < nr) {
> +            pci_default_write_config(dev, PCI_SUBORDINATE_BUS, nr, 1);
> +        }
> +    }
> +} > +
>   static void s390_pcihost_plug(HotplugHandler *hotplug_dev, DeviceState *dev,
>                                 Error **errp)
>   {
> @@ -851,26 +866,21 @@ static void s390_pcihost_plug(HotplugHandler *hotplug_dev, DeviceState *dev,
>       S390PCIBusDevice *pbdev = NULL;
>   
>       if (object_dynamic_cast(OBJECT(dev), TYPE_PCI_BRIDGE)) {
> -        BusState *bus;
>           PCIBridge *pb = PCI_BRIDGE(dev);
> -        PCIDevice *pdev = PCI_DEVICE(dev);
>   
> +        pdev = PCI_DEVICE(dev);
>           pci_bridge_map_irq(pb, dev->id, s390_pci_map_irq);
>           pci_setup_iommu(&pb->sec_bus, s390_pci_dma_iommu, s);
>   
> -        bus = BUS(&pb->sec_bus);
> -        qbus_set_hotplug_handler(bus, DEVICE(s), errp);
> +        qbus_set_hotplug_handler(BUS(&pb->sec_bus), DEVICE(s), errp);
>   
>           if (dev->hotplugged) {
>               pci_default_write_config(pdev, PCI_PRIMARY_BUS,
>                                        pci_dev_bus_num(pdev), 1);
>               s->bus_no += 1;
>               pci_default_write_config(pdev, PCI_SECONDARY_BUS, s->bus_no, 1);
> -            do {
> -                pdev = pci_get_bus(pdev)->parent_dev;
> -                pci_default_write_config(pdev, PCI_SUBORDINATE_BUS,
> -                                         s->bus_no, 1);
> -            } while (pci_get_bus(pdev) && pci_dev_bus_num(pdev));
> +
> +            s390_pci_update_subordinate(pdev, s->bus_no);
>           }
>       } else if (object_dynamic_cast(OBJECT(dev), TYPE_PCI_DEVICE)) {
>           pdev = PCI_DEVICE(dev);
> 

Looks good to me...

Reviewed-by: Collin Walling <walling@linux.ibm.com>

Side note: unrelated to the changes here -- and if you can clarify for
me -- any idea why we do s->bus_no += 1? This throws me off a bit and
begs me to ask what exactly is the S390pciState object suppose to
represent? (My guess is that it is representative of the entire PCI
topology, and we increment the bus_no to denote the subordinate bus
number?)

(let me know if these kind of discussions are too noisy and deemed
inappropriate for the mailing list, and I'll start pestering you off-
list instead)