diff --git a/sys/dev/pci/pci_pci.c b/sys/dev/pci/pci_pci.c
index 99424bac26e3..31d97f814239 100644
--- a/sys/dev/pci/pci_pci.c
+++ b/sys/dev/pci/pci_pci.c
@@ -445,3 +445,84 @@ pcib_route_interrupt(device_t pcib, device_t dev, int pin)
     }
     return(intnum);
 }
+
+/*
+ * Try to read the bus number of a host-PCI bridge using appropriate config
+ * registers.
+ */
+int
+host_pcib_get_busno(pci_read_config_fn read_config, int bus, int slot, int func,
+    u_int8_t *busnum)
+{
+	u_int32_t id;
+
+	id = read_config(bus, slot, func, PCIR_DEVVENDOR, 4);
+	if (id == 0xffff)
+		return (0);
+
+	switch (id) {
+	case 0x12258086:
+		/* Intel 824?? */
+		/* XXX This is a guess */
+		/* *busnum = read_config(bus, slot, func, 0x41, 1); */
+		*busnum = bus;
+		break;
+	case 0x84c48086:
+		/* Intel 82454KX/GX (Orion) */
+		*busnum = read_config(bus, slot, func, 0x4a, 1);
+		break;
+	case 0x84ca8086:
+		/*
+		 * For the 450nx chipset, there is a whole bundle of
+		 * things pretending to be host bridges. The MIOC will 
+		 * be seen first and isn't really a pci bridge (the
+		 * actual busses are attached to the PXB's). We need to 
+		 * read the registers of the MIOC to figure out the
+		 * bus numbers for the PXB channels.
+		 *
+		 * Since the MIOC doesn't have a pci bus attached, we
+		 * pretend it wasn't there.
+		 */
+		return (0);
+	case 0x84cb8086:
+		switch (slot) {
+		case 0x12:
+			/* Intel 82454NX PXB#0, Bus#A */
+			*busnum = read_config(bus, 0, func, 0xd0, 1);
+			break;
+		case 0x13:
+			/* Intel 82454NX PXB#0, Bus#B */
+			*busnum = read_config(bus, 0, func, 0xd1, 1) + 1;
+			break;
+		case 0x14:
+			/* Intel 82454NX PXB#1, Bus#A */
+			*busnum = read_config(bus, 0, func, 0xd3, 1);
+			break;
+		case 0x15:
+			/* Intel 82454NX PXB#1, Bus#B */
+			*busnum = read_config(bus, 0, func, 0xd4, 1) + 1;
+			break;
+		}
+		break;
+
+		/* ServerWorks -- vendor 0x1166 */
+	case 0x00051166:
+	case 0x00061166:
+	case 0x00081166:
+	case 0x00091166:
+	case 0x00101166:
+	case 0x00111166:
+	case 0x00171166:
+	case 0x01011166:
+	case 0x010f1014:
+	case 0x02011166:
+	case 0x03021014:
+		*busnum = read_config(bus, slot, func, 0x44, 1);
+		break;
+	default:
+		/* Don't know how to read bus number. */
+		return 0;
+	}
+
+	return 1;
+}
diff --git a/sys/dev/pci/pcib_private.h b/sys/dev/pci/pcib_private.h
index ec25172a144f..b6ef8b85fed1 100644
--- a/sys/dev/pci/pcib_private.h
+++ b/sys/dev/pci/pcib_private.h
@@ -58,6 +58,10 @@ struct pcib_softc
     u_int8_t	seclat;		/* secondary bus latency timer */
 };
 
+typedef u_int32_t pci_read_config_fn(int b, int s, int f, int reg, int width);
+
+int		host_pcib_get_busno(pci_read_config_fn read_config, int bus,
+    int slot, int func, u_int8_t *busnum);
 int		pcib_attach(device_t dev);
 void		pcib_attach_common(device_t dev);
 int		pcib_read_ivar(device_t dev, device_t child, int which, uintptr_t *result);