Re: [RFC PATCH v3 16/31] hw/pci: Plumb _UID through host bridges

[Jonathan Cameron <Jonathan.Cameron@Huawei.com>]

On Mon, 1 Feb 2021 16:59:33 -0800
Ben Widawsky <ben.widawsky@intel.com> wrote:

> Currently, QEMU makes _UID equivalent to the bus number (_BBN). While
> there is nothing wrong with doing it this way, CXL spec has a heavy
> reliance on _UID to identify host bridges and there is no link to the
> bus number. Having a distinct UID solves two problems. The first is it
> gets us around the limitation of 256 (current max bus number). The
> second is it allows us to replicate hardware configurations where bus
> number and uid aren't equivalent. The latter has benefits for our
> development and debugging using QEMU.
> 
> The other way to do this would be to implement the expanded bus
> numbering, but having an explicit uid makes more sense when trying to
> replicate real hardware configurations.
> 
> The QEMU commandline to utilize this would be:
>   -device pxb-cxl,id=cxl.0,bus="pcie.0",bus_nr=1,uid=x
> 
> Signed-off-by: Ben Widawsky <ben.widawsky@intel.com>
> 
> --
> 
> I'm guessing this patch will be somewhat controversial. For early CXL
> work, this can be dropped without too much heartache.

Whilst I'm not personally against, this maybe best to drop for now as you
say.

> ---
>  hw/i386/acpi-build.c                |  3 ++-
>  hw/pci-bridge/pci_expander_bridge.c | 19 +++++++++++++++++++
>  hw/pci/pci.c                        | 11 +++++++++++
>  include/hw/pci/pci.h                |  1 +
>  include/hw/pci/pci_bus.h            |  1 +
>  5 files changed, 34 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/i386/acpi-build.c b/hw/i386/acpi-build.c
> index cf6eb54c22..145a503e92 100644
> --- a/hw/i386/acpi-build.c
> +++ b/hw/i386/acpi-build.c
> @@ -1343,6 +1343,7 @@ build_dsdt(GArray *table_data, BIOSLinker *linker,
>          QLIST_FOREACH(bus, &bus->child, sibling) {
>              uint8_t bus_num = pci_bus_num(bus);
>              uint8_t numa_node = pci_bus_numa_node(bus);
> +            int32_t uid = pci_bus_uid(bus);
>  
>              /* look only for expander root buses */
>              if (!pci_bus_is_root(bus)) {
> @@ -1356,7 +1357,7 @@ build_dsdt(GArray *table_data, BIOSLinker *linker,
>              scope = aml_scope("\\_SB");
>              dev = aml_device("PC%.02X", bus_num);
>              aml_append(dev, aml_name_decl("_BBN", aml_int(bus_num)));
> -            init_pci_acpi(dev, bus_num, pci_bus_is_express(bus) ? PCIE : PCI);
> +            init_pci_acpi(dev, uid, pci_bus_is_express(bus) ? PCIE : PCI);
>  
>              if (numa_node != NUMA_NODE_UNASSIGNED) {
>                  aml_append(dev, aml_name_decl("_PXM", aml_int(numa_node)));
> diff --git a/hw/pci-bridge/pci_expander_bridge.c b/hw/pci-bridge/pci_expander_bridge.c
> index b42592e1ff..5021b60435 100644
> --- a/hw/pci-bridge/pci_expander_bridge.c
> +++ b/hw/pci-bridge/pci_expander_bridge.c
> @@ -67,6 +67,7 @@ struct PXBDev {
>  
>      uint8_t bus_nr;
>      uint16_t numa_node;
> +    int32_t uid;
>  };
>  
>  static PXBDev *convert_to_pxb(PCIDevice *dev)
> @@ -98,12 +99,20 @@ static uint16_t pxb_bus_numa_node(PCIBus *bus)
>      return pxb->numa_node;
>  }
>  
> +static int32_t pxb_bus_uid(PCIBus *bus)
> +{
> +    PXBDev *pxb = convert_to_pxb(bus->parent_dev);
> +
> +    return pxb->uid;
> +}
> +
>  static void pxb_bus_class_init(ObjectClass *class, void *data)
>  {
>      PCIBusClass *pbc = PCI_BUS_CLASS(class);
>  
>      pbc->bus_num = pxb_bus_num;
>      pbc->numa_node = pxb_bus_numa_node;
> +    pbc->uid = pxb_bus_uid;
>  }
>  
>  static const TypeInfo pxb_bus_info = {
> @@ -329,6 +338,7 @@ static Property pxb_dev_properties[] = {
>      /* Note: 0 is not a legal PXB bus number. */
>      DEFINE_PROP_UINT8("bus_nr", PXBDev, bus_nr, 0),
>      DEFINE_PROP_UINT16("numa_node", PXBDev, numa_node, NUMA_NODE_UNASSIGNED),
> +    DEFINE_PROP_INT32("uid", PXBDev, uid, -1),
>      DEFINE_PROP_END_OF_LIST(),
>  };
>  
> @@ -400,12 +410,21 @@ static const TypeInfo pxb_pcie_dev_info = {
>  
>  static void pxb_cxl_dev_realize(PCIDevice *dev, Error **errp)
>  {
> +    PXBDev *pxb = convert_to_pxb(dev);
> +
>      /* A CXL PXB's parent bus is still PCIe */
>      if (!pci_bus_is_express(pci_get_bus(dev))) {
>          error_setg(errp, "pxb-cxl devices cannot reside on a PCI bus");
>          return;
>      }
>  
> +    if (pxb->uid < 0) {
> +        error_setg(errp, "pxb-cxl devices must have a valid uid (0-2147483647)");
> +        return;
> +    }
> +
> +    /* FIXME: Check that uid doesn't collide with UIDs of other host bridges */
> +
>      pxb_dev_realize_common(dev, CXL, errp);
>  }
>  
> diff --git a/hw/pci/pci.c b/hw/pci/pci.c
> index adbe8aa260..bf019d91a0 100644
> --- a/hw/pci/pci.c
> +++ b/hw/pci/pci.c
> @@ -170,6 +170,11 @@ static uint16_t pcibus_numa_node(PCIBus *bus)
>      return NUMA_NODE_UNASSIGNED;
>  }
>  
> +static int32_t pcibus_uid(PCIBus *bus)
> +{
> +    return -1;
> +}
> +
>  static void pci_bus_class_init(ObjectClass *klass, void *data)
>  {
>      BusClass *k = BUS_CLASS(klass);
> @@ -184,6 +189,7 @@ static void pci_bus_class_init(ObjectClass *klass, void *data)
>  
>      pbc->bus_num = pcibus_num;
>      pbc->numa_node = pcibus_numa_node;
> +    pbc->uid = pcibus_uid;
>  }
>  
>  static const TypeInfo pci_bus_info = {
> @@ -530,6 +536,11 @@ int pci_bus_numa_node(PCIBus *bus)
>      return PCI_BUS_GET_CLASS(bus)->numa_node(bus);
>  }
>  
> +int pci_bus_uid(PCIBus *bus)
> +{
> +    return PCI_BUS_GET_CLASS(bus)->uid(bus);
> +}
> +
>  static int get_pci_config_device(QEMUFile *f, void *pv, size_t size,
>                                   const VMStateField *field)
>  {
> diff --git a/include/hw/pci/pci.h b/include/hw/pci/pci.h
> index bde3697bee..a46de48ccd 100644
> --- a/include/hw/pci/pci.h
> +++ b/include/hw/pci/pci.h
> @@ -463,6 +463,7 @@ static inline int pci_dev_bus_num(const PCIDevice *dev)
>  }
>  
>  int pci_bus_numa_node(PCIBus *bus);
> +int pci_bus_uid(PCIBus *bus);
>  void pci_for_each_device(PCIBus *bus, int bus_num,
>                           void (*fn)(PCIBus *bus, PCIDevice *d, void *opaque),
>                           void *opaque);
> diff --git a/include/hw/pci/pci_bus.h b/include/hw/pci/pci_bus.h
> index eb94e7e85c..3c9fbc55bb 100644
> --- a/include/hw/pci/pci_bus.h
> +++ b/include/hw/pci/pci_bus.h
> @@ -17,6 +17,7 @@ struct PCIBusClass {
>  
>      int (*bus_num)(PCIBus *bus);
>      uint16_t (*numa_node)(PCIBus *bus);
> +    int32_t (*uid)(PCIBus *bus);
>  };
>  
>  enum PCIBusFlags {