diff --git a/sys/arm/arm/gic_common.h b/sys/arm/arm/gic_common.h
--- a/sys/arm/arm/gic_common.h
+++ b/sys/arm/arm/gic_common.h
@@ -35,10 +35,12 @@
 	uint64_t size;
 };
 
-#define	GIC_IVAR_HW_REV		500
-#define	GIC_IVAR_BUS		501
-#define	GIC_IVAR_VGIC		502
-#define	GIC_IVAR_SUPPORT_LPIS	503
+enum {
+	GIC_IVAR_HW_REV = BUS_IVARS_GIC,
+	GIC_IVAR_BUS,
+	GIC_IVAR_VGIC,
+	GIC_IVAR_SUPPORT_LPIS
+};
 
 /* GIC_IVAR_BUS values */
 #define	GIC_BUS_UNKNOWN		0
diff --git a/sys/arm64/arm64/gic_v3_var.h b/sys/arm64/arm64/gic_v3_var.h
--- a/sys/arm64/arm64/gic_v3_var.h
+++ b/sys/arm64/arm64/gic_v3_var.h
@@ -109,10 +109,12 @@
 MALLOC_DECLARE(M_GIC_V3);
 
 /* ivars */
-#define	GICV3_IVAR_NIRQS	1000
-/* 1001 was GICV3_IVAR_REDIST_VADDR */
-#define	GICV3_IVAR_REDIST	1002
-#define	GICV3_IVAR_FLAGS	1003
+enum {
+	GICV3_IVAR_NIRQS = BUS_IVARS_GICV3,
+	_GICV3_IVAR_REDIST_VADDR,	/* unused */
+	GICV3_IVAR_REDIST,
+	GICV3_IVAR_FLAGS,
+};
 
 __BUS_ACCESSOR(gicv3, nirqs, GICV3, NIRQS, u_int);
 __BUS_ACCESSOR(gicv3, redist, GICV3, REDIST, void *);
diff --git a/sys/dev/acpica/acpivar.h b/sys/dev/acpica/acpivar.h
--- a/sys/dev/acpica/acpivar.h
+++ b/sys/dev/acpica/acpivar.h
@@ -275,11 +275,13 @@
  * interface compatibility with ISA drivers which can also
  * attach to ACPI.
  */
-#define ACPI_IVAR_HANDLE	0x100
-#define ACPI_IVAR_UNUSED	0x101	/* Unused/reserved. */
-#define ACPI_IVAR_PRIVATE	0x102
-#define ACPI_IVAR_FLAGS		0x103
-#define	ACPI_IVAR_DOMAIN	0x104
+enum {
+	ACPI_IVAR_HANDLE = BUS_IVARS_ACPI,
+	ACPI_IVAR_UNUSED,		/* Unused/reserved. */
+	ACPI_IVAR_PRIVATE,
+	ACPI_IVAR_FLAGS,
+	ACPI_IVAR_DOMAIN
+};
 
 /*
  * ad_domain NUMA domain special value.
diff --git a/sys/dev/atkbdc/atkbdcreg.h b/sys/dev/atkbdc/atkbdcreg.h
--- a/sys/dev/atkbdc/atkbdcreg.h
+++ b/sys/dev/atkbdc/atkbdcreg.h
@@ -215,7 +215,7 @@
 } atkbdc_softc_t; 
 
 enum kbdc_device_ivar {
-	KBDC_IVAR_VENDORID,
+	KBDC_IVAR_VENDORID = BUS_IVARS_PRIVATE,
 	KBDC_IVAR_SERIAL,
 	KBDC_IVAR_LOGICALID,
 	KBDC_IVAR_COMPATID, 
diff --git a/sys/dev/bhnd/bhnd.h b/sys/dev/bhnd/bhnd.h
--- a/sys/dev/bhnd/bhnd.h
+++ b/sys/dev/bhnd/bhnd.h
@@ -60,7 +60,7 @@
  * bhnd child instance variables
  */
 enum bhnd_device_vars {
-	BHND_IVAR_VENDOR,	/**< Designer's JEP-106 manufacturer ID. */
+	BHND_IVAR_VENDOR = BUS_IVARS_PRIVATE,	/**< Designer's JEP-106 manufacturer ID. */
 	BHND_IVAR_DEVICE,	/**< Part number */
 	BHND_IVAR_HWREV,	/**< Core revision */
 	BHND_IVAR_DEVICE_CLASS,	/**< Core class (@sa bhnd_devclass_t) */
diff --git a/sys/dev/fdc/fdcvar.h b/sys/dev/fdc/fdcvar.h
--- a/sys/dev/fdc/fdcvar.h
+++ b/sys/dev/fdc/fdcvar.h
@@ -69,7 +69,7 @@
 };
 
 enum fdc_device_ivars {
-	FDC_IVAR_FDUNIT,
+	FDC_IVAR_FDUNIT = BUS_IVARS_PRIVATE,
 	FDC_IVAR_FDTYPE,
 };
 
diff --git a/sys/dev/gpio/gpiobusvar.h b/sys/dev/gpio/gpiobusvar.h
--- a/sys/dev/gpio/gpiobusvar.h
+++ b/sys/dev/gpio/gpiobusvar.h
@@ -109,7 +109,7 @@
 };
 
 enum gpiobus_ivars {
-	GPIOBUS_IVAR_NPINS	= 10500,
+	GPIOBUS_IVAR_NPINS	= BUS_IVARS_GPIOBUS,
 	GPIOBUS_IVAR_PINS,
 };
 
diff --git a/sys/dev/hid/hidbus.h b/sys/dev/hid/hidbus.h
--- a/sys/dev/hid/hidbus.h
+++ b/sys/dev/hid/hidbus.h
@@ -27,7 +27,7 @@
 #define _HID_HIDBUS_H_
 
 enum {
-	HIDBUS_IVAR_USAGE,
+	HIDBUS_IVAR_USAGE = BUS_IVARS_PRIVATE,
 	HIDBUS_IVAR_INDEX,
 	HIDBUS_IVAR_FLAGS,
 #define	HIDBUS_FLAG_AUTOCHILD	(0<<1)	/* Child is autodiscovered */
diff --git a/sys/dev/iicbus/iicbus.h b/sys/dev/iicbus/iicbus.h
--- a/sys/dev/iicbus/iicbus.h
+++ b/sys/dev/iicbus/iicbus.h
@@ -57,7 +57,7 @@
 
 /* Value of 0x100 is reserved for ACPI_IVAR_HANDLE used by acpi_iicbus */
 enum {
-	IICBUS_IVAR_ADDR		/* Address or base address */
+	IICBUS_IVAR_ADDR = BUS_IVARS_PRIVATE	/* Address or base address */
 };
 
 #define IICBUS_ACCESSOR(A, B, T)					\
diff --git a/sys/dev/mii/miivar.h b/sys/dev/mii/miivar.h
--- a/sys/dev/mii/miivar.h
+++ b/sys/dev/mii/miivar.h
@@ -250,7 +250,7 @@
 	(*(p)->mii_funcs->pf_reset)(p)
 
 enum miibus_device_ivars {
-	MIIBUS_IVAR_FLAGS
+	MIIBUS_IVAR_FLAGS = BUS_IVARS_PRIVATE
 };
 
 /*
diff --git a/sys/dev/mmc/mmcbrvar.h b/sys/dev/mmc/mmcbrvar.h
--- a/sys/dev/mmc/mmcbrvar.h
+++ b/sys/dev/mmc/mmcbrvar.h
@@ -60,7 +60,7 @@
 #include "mmcbr_if.h"
 
 enum mmcbr_device_ivars {
-    MMCBR_IVAR_BUS_TYPE,
+    MMCBR_IVAR_BUS_TYPE = BUS_IVARS_PRIVATE,
     MMCBR_IVAR_BUS_MODE,
     MMCBR_IVAR_BUS_WIDTH,
     MMCBR_IVAR_CHIP_SELECT,
diff --git a/sys/dev/mmc/mmcvar.h b/sys/dev/mmc/mmcvar.h
--- a/sys/dev/mmc/mmcvar.h
+++ b/sys/dev/mmc/mmcvar.h
@@ -56,7 +56,7 @@
 #define DEV_MMC_MMCVAR_H
 
 enum mmc_device_ivars {
-    MMC_IVAR_SPEC_VERS,
+    MMC_IVAR_SPEC_VERS = BUS_IVARS_PRIVATE,
     MMC_IVAR_DSR_IMP,
     MMC_IVAR_MEDIA_SIZE,
     MMC_IVAR_RCA,
diff --git a/sys/dev/nvdimm/nvdimm_var.h b/sys/dev/nvdimm/nvdimm_var.h
--- a/sys/dev/nvdimm/nvdimm_var.h
+++ b/sys/dev/nvdimm/nvdimm_var.h
@@ -79,7 +79,7 @@
 typedef uint32_t nfit_handle_t;
 
 enum nvdimm_acpi_ivar {
-	NVDIMM_ROOT_IVAR_ACPI_HANDLE,
+	NVDIMM_ROOT_IVAR_ACPI_HANDLE = BUS_IVARS_PRIVATE,
 	NVDIMM_ROOT_IVAR_DEVICE_HANDLE,
 	NVDIMM_ROOT_IVAR_MAX,
 };
diff --git a/sys/dev/ow/ow.h b/sys/dev/ow/ow.h
--- a/sys/dev/ow/ow.h
+++ b/sys/dev/ow/ow.h
@@ -27,7 +27,7 @@
 #define DEV_OW_OW_H 1
 
 enum ow_device_ivars {
-	OW_IVAR_FAMILY,
+	OW_IVAR_FAMILY = BUS_IVARS_PRIVATE,
 	OW_IVAR_ROMID
 };
 
diff --git a/sys/dev/pccard/pccardvar.h b/sys/dev/pccard/pccardvar.h
--- a/sys/dev/pccard/pccardvar.h
+++ b/sys/dev/pccard/pccardvar.h
@@ -191,7 +191,7 @@
 
 /* ivar interface */
 enum {
-	PCCARD_IVAR_ETHADDR,	/* read ethernet address from CIS tupple */
+	PCCARD_IVAR_ETHADDR = BUS_IVARS_PRIVATE, /* read ethernet address from CIS tupple */
 	PCCARD_IVAR_VENDOR,
 	PCCARD_IVAR_PRODUCT,
 	PCCARD_IVAR_PRODEXT,
diff --git a/sys/dev/pci/pcivar.h b/sys/dev/pci/pcivar.h
--- a/sys/dev/pci/pcivar.h
+++ b/sys/dev/pci/pcivar.h
@@ -340,7 +340,7 @@
 #include "pci_if.h"
 
 enum pci_device_ivars {
-    PCI_IVAR_SUBVENDOR,
+    PCI_IVAR_SUBVENDOR = BUS_IVARS_PRIVATE,
     PCI_IVAR_SUBDEVICE,
     PCI_IVAR_VENDOR,
     PCI_IVAR_DEVICE,
@@ -414,7 +414,7 @@
 
 /*typedef enum pci_device_ivars pcib_device_ivars;*/
 enum pcib_device_ivars {
-	PCIB_IVAR_DOMAIN,
+	PCIB_IVAR_DOMAIN = BUS_IVARS_PRIVATE,
 	PCIB_IVAR_BUS
 };
 
diff --git a/sys/dev/ppbus/ppbconf.h b/sys/dev/ppbus/ppbconf.h
--- a/sys/dev/ppbus/ppbconf.h
+++ b/sys/dev/ppbus/ppbconf.h
@@ -178,7 +178,9 @@
 /*
  * List of IVARS available to ppb device drivers
  */
-#define PPBUS_IVAR_MODE 0
+enum {
+	PPBUS_IVAR_MODE = BUS_IVARS_PRIVATE
+};
 
 /* other fields are reserved to the ppbus internals */
 
@@ -208,9 +210,11 @@
 #define EPP_1_7		0x1
 
 /* Parallel Port Chipset IVARS */		/* elsewhere XXX */
-#define PPC_IVAR_EPP_PROTO	0
-#define PPC_IVAR_LOCK		1
-#define PPC_IVAR_INTR_HANDLER	2
+enum {
+	PPC_IVAR_EPP_PROTO = BUS_IVARS_PRIVATE,
+	PPC_IVAR_LOCK,
+	PPC_IVAR_INTR_HANDLER
+};
 
 /*
  * Maximum size of the PnP info string
diff --git a/sys/dev/puc/puc_bus.h b/sys/dev/puc/puc_bus.h
--- a/sys/dev/puc/puc_bus.h
+++ b/sys/dev/puc/puc_bus.h
@@ -32,8 +32,10 @@
 #include <sys/serial.h>
 #include <serdev_if.h>
 
-#define	PUC_IVAR_CLOCK		0
-#define	PUC_IVAR_TYPE		1
+enum {
+	PUC_IVAR_CLOCK = BUS_IVARS_PRIVATE,
+	PUC_IVAR_TYPE
+};
 
 /* Port types. */
 #define	PUC_TYPE_SERIAL		1
diff --git a/sys/dev/pwm/pwmbus.h b/sys/dev/pwm/pwmbus.h
--- a/sys/dev/pwm/pwmbus.h
+++ b/sys/dev/pwm/pwmbus.h
@@ -38,7 +38,7 @@
 };
 
 enum {
-	PWMBUS_IVAR_CHANNEL,	/* Channel used by child dev */
+	PWMBUS_IVAR_CHANNEL = BUS_IVARS_PRIVATE, /* Channel used by child dev */
 };
 
 #define PWMBUS_ACCESSOR(A, B, T)					\
diff --git a/sys/dev/quicc/quicc_bus.h b/sys/dev/quicc/quicc_bus.h
--- a/sys/dev/quicc/quicc_bus.h
+++ b/sys/dev/quicc/quicc_bus.h
@@ -29,9 +29,11 @@
 #ifndef _DEV_QUICC_BUS_H_
 #define	_DEV_QUICC_BUS_H_
 
-#define	QUICC_IVAR_CLOCK	1	/* The CPM clock. */
-#define	QUICC_IVAR_BRGCLK	2	/* The BRG clock affected by SCCR. */
-#define	QUICC_IVAR_DEVTYPE	3
+enum {
+	QUICC_IVAR_CLOCK = BUS_IVARS_PRIVATE,	/* The CPM clock. */
+	QUICC_IVAR_BRGCLK,		/* The BRG clock affected by SCCR. */
+	QUICC_IVAR_DEVTYPE
+};
 
 /* Device types. */
 #define	QUICC_DEVTYPE_SCC	1
diff --git a/sys/dev/scc/scc_bus.h b/sys/dev/scc/scc_bus.h
--- a/sys/dev/scc/scc_bus.h
+++ b/sys/dev/scc/scc_bus.h
@@ -32,12 +32,14 @@
 #include <sys/serial.h>
 #include <serdev_if.h>
 
-#define	SCC_IVAR_CHANNEL	0
-#define	SCC_IVAR_CLASS		1
-#define	SCC_IVAR_CLOCK		2
-#define	SCC_IVAR_MODE		3
-#define	SCC_IVAR_REGSHFT	4
-#define	SCC_IVAR_HWMTX		5
+enum {
+	SCC_IVAR_CHANNEL = BUS_IVARS_PRIVATE,
+	SCC_IVAR_CLASS,
+	SCC_IVAR_CLOCK,
+	SCC_IVAR_MODE,
+	SCC_IVAR_REGSHFT,
+	SCC_IVAR_HWMTX
+};
 
 /* Hardware class -- the SCC type. */
 #define	SCC_CLASS_UNUSED	0
diff --git a/sys/dev/sdio/sdiob.h b/sys/dev/sdio/sdiob.h
--- a/sys/dev/sdio/sdiob.h
+++ b/sys/dev/sdio/sdiob.h
@@ -64,7 +64,7 @@
 #ifdef _SYS_BUS_H_
 /* Ivars for sdiob. */
 enum sdiob_dev_enum {
-	SDIOB_IVAR_SUPPORT_MULTIBLK,
+	SDIOB_IVAR_SUPPORT_MULTIBLK = BUS_IVARS_PRIVATE,
 	SDIOB_IVAR_FUNCTION,
 	SDIOB_IVAR_FUNCNUM,
 	SDIOB_IVAR_CLASS,
diff --git a/sys/dev/smbus/smbconf.h b/sys/dev/smbus/smbconf.h
--- a/sys/dev/smbus/smbconf.h
+++ b/sys/dev/smbus/smbconf.h
@@ -75,7 +75,7 @@
  * ivars codes
  */
 enum smbus_ivars {
-    SMBUS_IVAR_ADDR,	/* slave address of the device */
+	SMBUS_IVAR_ADDR = BUS_IVARS_PRIVATE, /* slave address of the device */
 };
 
 int	smbus_request_bus(device_t, device_t, int);
diff --git a/sys/dev/sound/pci/hda/hdac.h b/sys/dev/sound/pci/hda/hdac.h
--- a/sys/dev/sound/pci/hda/hdac.h
+++ b/sys/dev/sound/pci/hda/hdac.h
@@ -982,7 +982,7 @@
  ****************************************************************************/
 
 enum hdac_device_ivars {
-    HDA_IVAR_CODEC_ID,
+    HDA_IVAR_CODEC_ID = BUS_IVARS_PRIVATE,
     HDA_IVAR_NODE_ID,
     HDA_IVAR_VENDOR_ID,
     HDA_IVAR_DEVICE_ID,
diff --git a/sys/dev/spibus/spibusvar.h b/sys/dev/spibus/spibusvar.h
--- a/sys/dev/spibus/spibusvar.h
+++ b/sys/dev/spibus/spibusvar.h
@@ -48,7 +48,7 @@
 #define	SPIBUS_CS_HIGH	(1U << 31)
 
 enum {
-	SPIBUS_IVAR_CS,		/* chip select that we're on */
+	SPIBUS_IVAR_CS = BUS_IVARS_PRIVATE,	/* chip select that we're on */
 	SPIBUS_IVAR_MODE,	/* SPI mode (0-3) */
 	SPIBUS_IVAR_CLOCK,	/* maximum clock freq for device */
 	SPIBUS_IVAR_CS_DELAY,	/* delay in microseconds after toggling chip select */
diff --git a/sys/dev/superio/superio.h b/sys/dev/superio/superio.h
--- a/sys/dev/superio/superio.h
+++ b/sys/dev/superio/superio.h
@@ -60,7 +60,7 @@
     int ldn);
 
 enum superio_ivars {
-	SUPERIO_IVAR_LDN =	10600,
+	SUPERIO_IVAR_LDN =	BUS_IVARS_SUPERIO,
 	SUPERIO_IVAR_TYPE,
 	SUPERIO_IVAR_IOBASE,
 	SUPERIO_IVAR_IOBASE2,
diff --git a/sys/dev/virtio/virtio.h b/sys/dev/virtio/virtio.h
--- a/sys/dev/virtio/virtio.h
+++ b/sys/dev/virtio/virtio.h
@@ -53,13 +53,15 @@
 /*
  * VirtIO instance variables indices.
  */
-#define VIRTIO_IVAR_DEVTYPE		1
-#define VIRTIO_IVAR_FEATURE_DESC	2
-#define VIRTIO_IVAR_VENDOR		3
-#define VIRTIO_IVAR_DEVICE		4
-#define VIRTIO_IVAR_SUBVENDOR		5
-#define VIRTIO_IVAR_SUBDEVICE		6
-#define VIRTIO_IVAR_MODERN		7
+enum {
+	VIRTIO_IVAR_DEVTYPE = BUS_IVARS_PRIVATE,
+	VIRTIO_IVAR_FEATURE_DESC,
+	VIRTIO_IVAR_VENDOR,
+	VIRTIO_IVAR_DEVICE,
+	VIRTIO_IVAR_SUBVENDOR,
+	VIRTIO_IVAR_SUBDEVICE,
+	VIRTIO_IVAR_MODERN
+};
 
 struct virtio_feature_desc {
 	uint64_t	 vfd_val;
diff --git a/sys/isa/isavar.h b/sys/isa/isavar.h
--- a/sys/isa/isavar.h
+++ b/sys/isa/isavar.h
@@ -103,7 +103,7 @@
 };
 
 enum isa_device_ivars {
-	ISA_IVAR_PORT,
+	ISA_IVAR_PORT = BUS_IVARS_PRIVATE,
 	ISA_IVAR_PORT_0 = ISA_IVAR_PORT,
 	ISA_IVAR_PORT_1,
 	ISA_IVAR_PORTSIZE,
diff --git a/sys/powerpc/ps3/ps3bus.h b/sys/powerpc/ps3/ps3bus.h
--- a/sys/powerpc/ps3/ps3bus.h
+++ b/sys/powerpc/ps3/ps3bus.h
@@ -29,7 +29,7 @@
 #define _POWERPC_PS3_PS3BUS_H
 
 enum {
-	PS3BUS_IVAR_BUS,
+	PS3BUS_IVAR_BUS = BUS_IVARS_PRIVATE,
 	PS3BUS_IVAR_DEVICE,
 	PS3BUS_IVAR_BUSTYPE,
 	PS3BUS_IVAR_DEVTYPE,
diff --git a/sys/powerpc/psim/iobusvar.h b/sys/powerpc/psim/iobusvar.h
--- a/sys/powerpc/psim/iobusvar.h
+++ b/sys/powerpc/psim/iobusvar.h
@@ -35,7 +35,7 @@
  */
 
 enum iobus_ivars {
-        IOBUS_IVAR_NODE,
+        IOBUS_IVAR_NODE = BUS_IVARS_PRIVATE,
         IOBUS_IVAR_NAME,
 	IOBUS_IVAR_NREGS,
 	IOBUS_IVAR_REGS,
diff --git a/sys/sys/bus.h b/sys/sys/bus.h
--- a/sys/sys/bus.h
+++ b/sys/sys/bus.h
@@ -297,6 +297,22 @@
 	INTR_POLARITY_LOW = 2
 };
 
+/**
+ * Bus drivers may maintain a set of bus-specific instance variables
+ * for each child device.  The BUS_READ_IVAR/BUS_WRITE_IVAR API can be
+ * used to access these variables using an index value.  Some index
+ * values are private to a single bus and should be defined in the
+ * private range.  Other index values are shared by multiple busses
+ * and must have the same meaning in all bus drivers.
+ */
+
+#define	BUS_IVARS_PRIVATE	0x0	/* private variables */
+#define	BUS_IVARS_ACPI		0x100
+#define	BUS_IVARS_GIC		500
+#define	BUS_IVARS_GICV3		1000
+#define	BUS_IVARS_GPIOBUS	10500
+#define	BUS_IVARS_SUPERIO	10600
+
 /**
  * CPU sets supported by bus_get_cpus().  Note that not all sets may be
  * supported for a given device.  If a request is not supported by a
diff --git a/sys/sys/cpu.h b/sys/sys/cpu.h
--- a/sys/sys/cpu.h
+++ b/sys/sys/cpu.h
@@ -35,10 +35,12 @@
  * CPU device support.
  */
 
-#define CPU_IVAR_PCPU		1
-#define CPU_IVAR_NOMINAL_MHZ	2
-#define CPU_IVAR_CPUID_SIZE	3
-#define CPU_IVAR_CPUID		4
+enum {
+	CPU_IVAR_PCPU = BUS_IVARS_PRIVATE,
+	CPU_IVAR_NOMINAL_MHZ,
+	CPU_IVAR_CPUID_SIZE,
+	CPU_IVAR_CPUID
+};
 
 static __inline struct pcpu *
 cpu_get_pcpu(device_t dev)
diff --git a/sys/x86/include/legacyvar.h b/sys/x86/include/legacyvar.h
--- a/sys/x86/include/legacyvar.h
+++ b/sys/x86/include/legacyvar.h
@@ -30,7 +30,7 @@
 #define	_X86_LEGACYVAR_H_
 
 enum legacy_device_ivars {
-	LEGACY_IVAR_PCIDOMAIN,
+	LEGACY_IVAR_PCIDOMAIN = BUS_IVARS_PRIVATE,
 	LEGACY_IVAR_PCIBUS,
 	LEGACY_IVAR_PCISLOT,
 	LEGACY_IVAR_PCIFUNC
diff --git a/sys/xen/xenbus/xenbusvar.h b/sys/xen/xenbus/xenbusvar.h
--- a/sys/xen/xenbus/xenbusvar.h
+++ b/sys/xen/xenbus/xenbusvar.h
@@ -55,7 +55,7 @@
 	/**
 	 * Path of this device node.
 	 */
-	XENBUS_IVAR_NODE,
+	XENBUS_IVAR_NODE = BUS_IVARS_PRIVATE,
 
 	/**
 	 * The device type (e.g. vif, vbd).