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

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

On Tue, Feb 02, 2021 at 03:00:56PM +0000, Jonathan Cameron wrote:
> 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).

Not sure I understand. You want more than 256 host bridges?

> The
> > second is it allows us to replicate hardware configurations where bus
> > number and uid aren't equivalent.

A bit more data on when this needs to be the case?

> 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>

However, if doing this how do we ensure UID is still unique?
What do we do for cases where UID was not specified?
One idea is to generate a string UID and just stick the bus #
in there.


> > --
> > 
> > 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)

As long as we are doing this, do we want to support a string uid too?
How about a 64 bit uid? Why not?


> > @@ -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 {