Re: [Qemu-devel] [PULL 16/17] hw/pci-bridge: format special OFW unit address for PXB host

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Nov 12, 2025 at 11:35:21PM +0100, Philippe Mathieu-Daudé wrote:
> Hi Michael and Marcel,
> 
> (old patch...)
> 
> On 26/6/15 15:46, Michael S. Tsirkin wrote:
> > From: Laszlo Ersek <lersek@redhat.com>
> > 
> > We have agreed that OpenFirmware device paths in the "bootorder" fw_cfg
> > file should follow the pattern
> > 
> >    /pci@i0cf8,%x/...
> > 
> > for devices that live behind an extra root bus. The extra root bus in
> > question is the %x'th among the extra root buses. (In other words, %x
> > gives the position of the affected extra root bus relative to the other
> > extra root buses, in bus_nr order.) %x starts at 1, and is formatted in
> > hex.
> > 
> > The portion of the unit address that comes before the comma is dynamically
> > taken from the main host bridge, similarly to sysbus_get_fw_dev_path().
> > 
> > Cc: Kevin O'Connor <kevin@koconnor.net>
> > Cc: Michael S. Tsirkin <mst@redhat.com>
> > Cc: Marcel Apfelbaum <marcel@redhat.com>
> > Signed-off-by: Laszlo Ersek <lersek@redhat.com>
> > Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> >   hw/pci-bridge/pci_expander_bridge.c | 53 +++++++++++++++++++++++++++++++++++++
> >   1 file changed, 53 insertions(+)
> > 
> > diff --git a/hw/pci-bridge/pci_expander_bridge.c b/hw/pci-bridge/pci_expander_bridge.c
> > index 70708ef..57f8a37 100644
> > --- a/hw/pci-bridge/pci_expander_bridge.c
> > +++ b/hw/pci-bridge/pci_expander_bridge.c
> > @@ -43,6 +43,8 @@ typedef struct PXBDev {
> >       uint16_t numa_node;
> >   } PXBDev;
> > +static GList *pxb_dev_list;
> 
> This list is static ...
> 
> > +
> >   #define TYPE_PXB_HOST "pxb-host"
> >   static int pxb_bus_num(PCIBus *bus)
> > @@ -89,12 +91,45 @@ static const char *pxb_host_root_bus_path(PCIHostState *host_bridge,
> >       return bus->bus_path;
> >   }
> > +static char *pxb_host_ofw_unit_address(const SysBusDevice *dev)
> > +{
> > +    const PCIHostState *pxb_host;
> > +    const PCIBus *pxb_bus;
> > +    const PXBDev *pxb_dev;
> > +    int position;
> > +    const DeviceState *pxb_dev_base;
> > +    const PCIHostState *main_host;
> > +    const SysBusDevice *main_host_sbd;
> > +
> > +    pxb_host = PCI_HOST_BRIDGE(dev);
> > +    pxb_bus = pxb_host->bus;
> > +    pxb_dev = PXB_DEV(pxb_bus->parent_dev);
> > +    position = g_list_index(pxb_dev_list, pxb_dev);
> > +    assert(position >= 0);
> 
> ... and for some reason when calling pxb_host_ofw_unit_address() twice
> it triggers the following:
> 
> Assertion failed: (position >= 0), function pxb_host_ofw_unit_address, file
> pci_expander_bridge.c, line 154.
> 
> Any idea why it got implemented this way and how to modify to
> avoid the assertion? All other devices implementing the
> SysBusDeviceClass::explicit_ofw_unit_address handler don't abort
> when being called multiple times, what is so particular here?
> 
> Thanks,
> 
> Phil.

IIUC what is weird about this one is that the ofw address depends on other
devices.


The fix is probably to scan the list once at machine done time
and save the position in the device.

> > +
> > +    pxb_dev_base = DEVICE(pxb_dev);
> > +    main_host = PCI_HOST_BRIDGE(pxb_dev_base->parent_bus->parent);
> > +    main_host_sbd = SYS_BUS_DEVICE(main_host);
> > +
> > +    if (main_host_sbd->num_mmio > 0) {
> > +        return g_strdup_printf(TARGET_FMT_plx ",%x",
> > +                               main_host_sbd->mmio[0].addr, position + 1);
> > +    }
> > +    if (main_host_sbd->num_pio > 0) {
> > +        return g_strdup_printf("i%04x,%x",
> > +                               main_host_sbd->pio[0], position + 1);
> > +    }
> > +    return NULL;
> > +}
> > +
> >   static void pxb_host_class_init(ObjectClass *class, void *data)
> >   {
> >       DeviceClass *dc = DEVICE_CLASS(class);
> > +    SysBusDeviceClass *sbc = SYS_BUS_DEVICE_CLASS(class);
> >       PCIHostBridgeClass *hc = PCI_HOST_BRIDGE_CLASS(class);
> >       dc->fw_name = "pci";
> > +    sbc->explicit_ofw_unit_address = pxb_host_ofw_unit_address;
> >       hc->root_bus_path = pxb_host_root_bus_path;
> >   }
> > @@ -149,6 +184,15 @@ static int pxb_map_irq_fn(PCIDevice *pci_dev, int pin)
> >       return pin - PCI_SLOT(pxb->devfn);
> >   }
> > +static gint pxb_compare(gconstpointer a, gconstpointer b)
> > +{
> > +    const PXBDev *pxb_a = a, *pxb_b = b;
> > +
> > +    return pxb_a->bus_nr < pxb_b->bus_nr ? -1 :
> > +           pxb_a->bus_nr > pxb_b->bus_nr ?  1 :
> > +           0;
> > +}
> > +
> >   static int pxb_dev_initfn(PCIDevice *dev)
> >   {
> >       PXBDev *pxb = PXB_DEV(dev);
> > @@ -192,9 +236,17 @@ static int pxb_dev_initfn(PCIDevice *dev)
> >                                  PCI_STATUS_66MHZ | PCI_STATUS_FAST_BACK);
> >       pci_config_set_class(dev->config, PCI_CLASS_BRIDGE_HOST);
> > +    pxb_dev_list = g_list_insert_sorted(pxb_dev_list, pxb, pxb_compare);
> >       return 0;
> >   }
> > +static void pxb_dev_exitfn(PCIDevice *pci_dev)
> > +{
> > +    PXBDev *pxb = PXB_DEV(pci_dev);
> > +
> > +    pxb_dev_list = g_list_remove(pxb_dev_list, pxb);
> > +}
> > +
> >   static Property pxb_dev_properties[] = {
> >       /* Note: 0 is not a legal a PXB bus number. */
> >       DEFINE_PROP_UINT8("bus_nr", PXBDev, bus_nr, 0),
> > @@ -208,6 +260,7 @@ static void pxb_dev_class_init(ObjectClass *klass, void *data)
> >       PCIDeviceClass *k = PCI_DEVICE_CLASS(klass);
> >       k->init = pxb_dev_initfn;
> > +    k->exit = pxb_dev_exitfn;
> >       k->vendor_id = PCI_VENDOR_ID_REDHAT;
> >       k->device_id = PCI_DEVICE_ID_REDHAT_PXB;
> >       k->class_id = PCI_CLASS_BRIDGE_HOST;