diff --git a/Makefile b/Makefile
index de11060..3f25a14 100644
--- a/Makefile
+++ b/Makefile
@@ -50,6 +50,7 @@ OBJS	+= builtin-run.o
 OBJS	+= builtin-setup.o
 OBJS	+= builtin-stop.o
 OBJS	+= builtin-version.o
+OBJS	+= devices.o
 OBJS	+= disk/core.o
 OBJS	+= framebuffer.o
 OBJS	+= guest_compat.o
diff --git a/devices.c b/devices.c
new file mode 100644
index 0000000..9f1941d
--- /dev/null
+++ b/devices.c
@@ -0,0 +1,86 @@
+#include "kvm/devices.h"
+#include "kvm/kvm.h"
+
+#include <linux/err.h>
+#include <linux/rbtree.h>
+
+struct device_bus {
+	struct rb_root	root;
+	int		dev_num;
+};
+
+static struct device_bus device_trees[DEVICE_BUS_MAX] = {
+	[0 ... (DEVICE_BUS_MAX - 1)] = { RB_ROOT, 0 },
+};
+
+int device__register(struct device_header *dev)
+{
+	struct device_bus *bus;
+	struct rb_node **node, *parent = NULL;
+
+	if (dev->bus_type >= DEVICE_BUS_MAX) {
+		pr_warning("Ignoring device registration on unknown bus %d\n",
+			   dev->bus_type);
+		return -EINVAL;
+	}
+
+	bus = &device_trees[dev->bus_type];
+	dev->dev_num = bus->dev_num++;
+
+	node = &bus->root.rb_node;
+	while (*node) {
+		int num = rb_entry(*node, struct device_header, node)->dev_num;
+		int result = dev->dev_num - num;
+
+		if (result < 0)
+			node = &((*node)->rb_left);
+		else if (result > 0)
+			node = &((*node)->rb_right);
+		else
+			return -EEXIST;
+	}
+
+	rb_link_node(&dev->node, parent, node);
+	rb_insert_color(&dev->node, &bus->root);
+	return 0;
+}
+
+struct device_header *device__find_dev(enum device_bus_type bus_type, u8 dev_num)
+{
+	struct rb_node *node;
+
+	if (bus_type >= DEVICE_BUS_MAX)
+		return ERR_PTR(-EINVAL);
+
+	node = device_trees[bus_type].root.rb_node;
+	while (node) {
+		struct device_header *dev = rb_entry(node, struct device_header,
+						     node);
+		if (dev_num < dev->dev_num) {
+			node = node->rb_left;
+		} else if (dev_num > dev->dev_num) {
+			node = node->rb_right;
+		} else {
+			return dev;
+		}
+	}
+
+	return NULL;
+}
+
+struct device_header *device__first_dev(enum device_bus_type bus_type)
+{
+	struct rb_node *node;
+
+	if (bus_type >= DEVICE_BUS_MAX)
+		return NULL;
+
+	node = rb_first(&device_trees[bus_type].root);
+	return node ? rb_entry(node, struct device_header, node) : NULL;
+}
+
+struct device_header *device__next_dev(struct device_header *dev)
+{
+	struct rb_node *node = rb_next(&dev->node);
+	return node ? rb_entry(node, struct device_header, node) : NULL;
+}
diff --git a/hw/pci-shmem.c b/hw/pci-shmem.c
index 4161335..00e5d93 100644
--- a/hw/pci-shmem.c
+++ b/hw/pci-shmem.c
@@ -1,3 +1,4 @@
+#include "kvm/devices.h"
 #include "kvm/pci-shmem.h"
 #include "kvm/virtio-pci-dev.h"
 #include "kvm/irq.h"
@@ -30,6 +31,11 @@ static struct pci_device_header pci_shmem_pci_device = {
 	.msix.pba_offset = cpu_to_le32(0x1001),		/* Use BAR 1 */
 };
 
+static struct device_header pci_shmem_device = {
+	.bus_type	= DEVICE_BUS_PCI,
+	.data		= &pci_shmem_pci_device,
+};
+
 /* registers for the Inter-VM shared memory device */
 enum ivshmem_registers {
 	INTRMASK = 0,
@@ -346,7 +352,7 @@ int shmem_parser(const struct option *opt, const char *arg, int unset)
 
 int pci_shmem__init(struct kvm *kvm)
 {
-	u8 dev, line, pin;
+	u8 line, pin;
 	char *mem;
 	int r;
 
@@ -354,7 +360,7 @@ int pci_shmem__init(struct kvm *kvm)
 		return 0;
 
 	/* Register good old INTx */
-	r = irq__register_device(PCI_DEVICE_ID_PCI_SHMEM, &dev, &pin, &line);
+	r = irq__register_device(PCI_DEVICE_ID_PCI_SHMEM, &pin, &line);
 	if (r < 0)
 		return r;
 
@@ -384,7 +390,7 @@ int pci_shmem__init(struct kvm *kvm)
 	pci_shmem_pci_device.bar[2] = cpu_to_le32(shmem_region->phys_addr | PCI_BASE_ADDRESS_SPACE_MEMORY);
 	pci_shmem_pci_device.bar_size[2] = shmem_region->size;
 
-	pci__register(&pci_shmem_pci_device, dev);
+	device__register(&pci_shmem_device);
 
 	/* Open shared memory and plug it into the guest */
 	mem = setup_shmem(shmem_region->handle, shmem_region->size,
diff --git a/hw/vesa.c b/hw/vesa.c
index a211491..33a675f 100644
--- a/hw/vesa.c
+++ b/hw/vesa.c
@@ -1,5 +1,6 @@
 #include "kvm/vesa.h"
 
+#include "kvm/devices.h"
 #include "kvm/virtio-pci-dev.h"
 #include "kvm/framebuffer.h"
 #include "kvm/kvm-cpu.h"
@@ -44,19 +45,24 @@ static struct pci_device_header vesa_pci_device = {
 	.bar_size[1]		= VESA_MEM_SIZE,
 };
 
+static struct device_header vesa_device = {
+	.bus_type	= DEVICE_BUS_PCI,
+	.data		= &vesa_pci_device,
+};
+
 static struct framebuffer vesafb;
 
 struct framebuffer *vesa__init(struct kvm *kvm)
 {
 	u16 vesa_base_addr;
-	u8 dev, line, pin;
+	u8 line, pin;
 	char *mem;
 	int r;
 
 	if (!kvm->cfg.vnc && !kvm->cfg.sdl)
 		return NULL;
 
-	r = irq__register_device(PCI_DEVICE_ID_VESA, &dev, &pin, &line);
+	r = irq__register_device(PCI_DEVICE_ID_VESA, &pin, &line);
 	if (r < 0)
 		return ERR_PTR(r);
 
@@ -68,7 +74,7 @@ struct framebuffer *vesa__init(struct kvm *kvm)
 	vesa_pci_device.irq_line	= line;
 	vesa_base_addr			= (u16)r;
 	vesa_pci_device.bar[0]		= cpu_to_le32(vesa_base_addr | PCI_BASE_ADDRESS_SPACE_IO);
-	pci__register(&vesa_pci_device, dev);
+	device__register(&vesa_device);
 
 	mem = mmap(NULL, VESA_MEM_SIZE, PROT_RW, MAP_ANON_NORESERVE, -1, 0);
 	if (mem == MAP_FAILED)
diff --git a/include/kvm/devices.h b/include/kvm/devices.h
new file mode 100644
index 0000000..c5de3de
--- /dev/null
+++ b/include/kvm/devices.h
@@ -0,0 +1,27 @@
+#ifndef KVM__DEVICES_H
+#define KVM__DEVICES_H
+
+#include <linux/rbtree.h>
+#include <linux/types.h>
+
+enum device_bus_type {
+	DEVICE_BUS_PCI,
+	DEVICE_BUS_MMIO,
+	DEVICE_BUS_MAX,
+};
+
+struct device_header {
+	enum device_bus_type	bus_type;
+	void			*data;
+	int			dev_num;
+	struct rb_node		node;
+};
+
+int device__register(struct device_header *dev);
+struct device_header *device__find_dev(enum device_bus_type bus_type,
+				       u8 dev_num);
+
+struct device_header *device__first_dev(enum device_bus_type bus_type);
+struct device_header *device__next_dev(struct device_header *dev);
+
+#endif /* KVM__DEVICES_H */
diff --git a/include/kvm/irq.h b/include/kvm/irq.h
index 43fecaf..5c1274b 100644
--- a/include/kvm/irq.h
+++ b/include/kvm/irq.h
@@ -22,7 +22,7 @@ struct pci_dev {
 	struct list_head	lines;
 };
 
-int irq__register_device(u32 dev, u8 *num, u8 *pin, u8 *line);
+int irq__register_device(u32 dev, u8 *pin, u8 *line);
 
 struct rb_node *irq__get_pci_tree(void);
 
diff --git a/include/kvm/pci.h b/include/kvm/pci.h
index 26639b5..3da3811 100644
--- a/include/kvm/pci.h
+++ b/include/kvm/pci.h
@@ -9,7 +9,6 @@
 #include "kvm/kvm.h"
 #include "kvm/msi.h"
 
-#define PCI_MAX_DEVICES		256
 /*
  * PCI Configuration Mechanism #1 I/O ports. See Section 3.7.4.1.
  * ("Configuration Mechanism #1") of the PCI Local Bus Specification 2.1 for
@@ -86,7 +85,6 @@ struct pci_device_header {
 
 int pci__init(struct kvm *kvm);
 int pci__exit(struct kvm *kvm);
-int pci__register(struct pci_device_header *dev, u8 dev_num);
 struct pci_device_header *pci__find_dev(u8 dev_num);
 u32 pci_get_io_space_block(u32 size);
 void pci__config_wr(struct kvm *kvm, union pci_config_address addr, void *data, int size);
diff --git a/include/kvm/virtio-mmio.h b/include/kvm/virtio-mmio.h
index e0ede3c..983c8fc 100644
--- a/include/kvm/virtio-mmio.h
+++ b/include/kvm/virtio-mmio.h
@@ -47,6 +47,7 @@ struct virtio_mmio {
 	struct kvm		*kvm;
 	u8			irq;
 	struct virtio_mmio_hdr	hdr;
+	struct device_header	dev_hdr;
 	struct virtio_mmio_ioevent_param ioeventfds[VIRTIO_MMIO_MAX_VQ];
 };
 
diff --git a/include/kvm/virtio-pci.h b/include/kvm/virtio-pci.h
index 44130e0..6d9a558 100644
--- a/include/kvm/virtio-pci.h
+++ b/include/kvm/virtio-pci.h
@@ -1,6 +1,7 @@
 #ifndef KVM__VIRTIO_PCI_H
 #define KVM__VIRTIO_PCI_H
 
+#include "kvm/devices.h"
 #include "kvm/pci.h"
 
 #include <linux/types.h>
@@ -19,6 +20,7 @@ struct virtio_pci_ioevent_param {
 
 struct virtio_pci {
 	struct pci_device_header pci_hdr;
+	struct device_header	dev_hdr;
 	void			*dev;
 
 	u16			base_addr;
diff --git a/pci.c b/pci.c
index c77d3cc..8d3732d 100644
--- a/pci.c
+++ b/pci.c
@@ -1,3 +1,4 @@
+#include "kvm/devices.h"
 #include "kvm/pci.h"
 #include "kvm/ioport.h"
 #include "kvm/util.h"
@@ -8,8 +9,6 @@
 
 #define PCI_BAR_OFFSET(b)		(offsetof(struct pci_device_header, bar[b]))
 
-static struct pci_device_header		*pci_devices[PCI_MAX_DEVICES];
-
 static union pci_config_address		pci_config_address;
 
 /* This is within our PCI gap - in an unused area.
@@ -63,20 +62,13 @@ static struct ioport_operations pci_config_address_ops = {
 
 static bool pci_device_exists(u8 bus_number, u8 device_number, u8 function_number)
 {
-	struct pci_device_header *dev;
-
 	if (pci_config_address.bus_number != bus_number)
 		return false;
 
 	if (pci_config_address.function_number != function_number)
 		return false;
 
-	if (device_number >= PCI_MAX_DEVICES)
-		return false;
-
-	dev = pci_devices[device_number];
-
-	return dev != NULL;
+	return !IS_ERR_OR_NULL(device__find_dev(DEVICE_BUS_PCI, device_number));
 }
 
 static bool pci_config_data_out(struct ioport *ioport, struct kvm *kvm, u16 port, void *data, int size)
@@ -121,12 +113,13 @@ void pci__config_wr(struct kvm *kvm, union pci_config_address addr, void *data,
 
 		offset = addr.w & 0xff;
 		if (offset < sizeof(struct pci_device_header)) {
-			void *p = pci_devices[dev_num];
+			void *p = device__find_dev(DEVICE_BUS_PCI, dev_num)->data;
+			struct pci_device_header *hdr = p;
 			u8 bar = (offset - PCI_BAR_OFFSET(0)) / (sizeof(u32));
 			u32 sz = PCI_IO_SIZE;
 
-			if (bar < 6 && pci_devices[dev_num]->bar_size[bar])
-				sz = pci_devices[dev_num]->bar_size[bar];
+			if (bar < 6 && hdr->bar_size[bar])
+				sz = hdr->bar_size[bar];
 
 			/*
 			 * If the kernel masks the BAR it would expect to find the
@@ -158,7 +151,7 @@ void pci__config_rd(struct kvm *kvm, union pci_config_address addr, void *data,
 
 		offset = addr.w & 0xff;
 		if (offset < sizeof(struct pci_device_header)) {
-			void *p = pci_devices[dev_num];
+			void *p = device__find_dev(DEVICE_BUS_PCI, dev_num)->data;
 
 			memcpy(data, p + offset, size);
 		} else {
@@ -169,22 +162,14 @@ void pci__config_rd(struct kvm *kvm, union pci_config_address addr, void *data,
 	}
 }
 
-int pci__register(struct pci_device_header *dev, u8 dev_num)
-{
-	if (dev_num >= PCI_MAX_DEVICES)
-		return -ENOSPC;
-
-	pci_devices[dev_num] = dev;
-
-	return 0;
-}
-
 struct pci_device_header *pci__find_dev(u8 dev_num)
 {
-	if (dev_num >= PCI_MAX_DEVICES)
-		return ERR_PTR(-EOVERFLOW);
+	struct device_header *hdr = device__find_dev(DEVICE_BUS_PCI, dev_num);
 
-	return pci_devices[dev_num];
+	if (IS_ERR_OR_NULL(hdr))
+		return NULL;
+
+	return hdr->data;
 }
 
 int pci__init(struct kvm *kvm)
diff --git a/powerpc/irq.c b/powerpc/irq.c
index e89fa3b..ae9da50 100644
--- a/powerpc/irq.c
+++ b/powerpc/irq.c
@@ -8,6 +8,7 @@
  * by the Free Software Foundation.
  */
 
+#include "kvm/devices.h"
 #include "kvm/irq.h"
 #include "kvm/kvm.h"
 #include "kvm/util.h"
@@ -31,15 +32,8 @@
  * generic & cope with multiple PPC platform types.
  */
 
-static int pci_devs = 0;
-
-int irq__register_device(u32 dev, u8 *num, u8 *pin, u8 *line)
+int irq__register_device(u32 dev, u8 *pin, u8 *line)
 {
-	if (pci_devs >= PCI_MAX_DEVICES)
-		die("Hit PCI device limit!\n");
-
-	*num = pci_devs++;
-
 	*pin = 1;
 	/*
 	 * Have I said how nasty I find this?  Line should be dontcare... PHB
diff --git a/powerpc/spapr_pci.c b/powerpc/spapr_pci.c
index 9f0ce7a..5f4016c 100644
--- a/powerpc/spapr_pci.c
+++ b/powerpc/spapr_pci.c
@@ -302,7 +302,7 @@ int spapr_populate_pci_devices(struct kvm *kvm,
 	/* Populate PCI devices and allocate IRQs */
 	devices = 0;
 
-	for (devid = 0; devid < PCI_MAX_DEVICES; devid++) {
+	for (devid = 0; devid < KVM_MAX_DEVICES; devid++) {
 		uint32_t *irqmap = interrupt_map[devices];
 		struct pci_device_header *hdr = pci__find_dev(devid);
 
diff --git a/virtio/mmio.c b/virtio/mmio.c
index fe42e95..f94c54f 100644
--- a/virtio/mmio.c
+++ b/virtio/mmio.c
@@ -1,3 +1,4 @@
+#include "kvm/devices.h"
 #include "kvm/virtio-mmio.h"
 #include "kvm/ioeventfd.h"
 #include "kvm/ioport.h"
@@ -218,7 +219,7 @@ int virtio_mmio_init(struct kvm *kvm, void *dev, struct virtio_device *vdev,
 		     int device_id, int subsys_id, int class)
 {
 	struct virtio_mmio *vmmio = vdev->virtio;
-	u8 device, pin, line;
+	u8 pin, line;
 
 	vmmio->addr	= virtio_mmio_get_io_space_block(VIRTIO_MMIO_IO_SIZE);
 	vmmio->kvm	= kvm;
@@ -235,9 +236,15 @@ int virtio_mmio_init(struct kvm *kvm, void *dev, struct virtio_device *vdev,
 		.queue_num_max	= 256,
 	};
 
-	if (irq__register_device(subsys_id, &device, &pin, &line) < 0)
+	if (irq__register_device(subsys_id, &pin, &line) < 0)
 		return -1;
 	vmmio->irq = line;
+	vmmio->dev_hdr = (struct device_header) {
+		.bus_type	= DEVICE_BUS_MMIO,
+		.data		= vmmio,
+	};
+
+	device__register(&vmmio->dev_hdr);
 
 	/*
 	 * Instantiate guest virtio-mmio devices using kernel command line
diff --git a/virtio/pci.c b/virtio/pci.c
index adc8efc..3d3e599 100644
--- a/virtio/pci.c
+++ b/virtio/pci.c
@@ -306,7 +306,7 @@ int virtio_pci__init(struct kvm *kvm, void *dev, struct virtio_device *vdev,
 		     int device_id, int subsys_id, int class)
 {
 	struct virtio_pci *vpci = vdev->virtio;
-	u8 pin, line, ndev;
+	u8 pin, line;
 	int r;
 
 	vpci->dev = dev;
@@ -343,6 +343,11 @@ int virtio_pci__init(struct kvm *kvm, void *dev, struct virtio_device *vdev,
 		.bar_size[3]		= PCI_IO_SIZE,
 	};
 
+	vpci->dev_hdr = (struct device_header) {
+		.bus_type		= DEVICE_BUS_PCI,
+		.data			= &vpci->pci_hdr,
+	};
+
 	vpci->pci_hdr.msix.cap = PCI_CAP_ID_MSIX;
 	vpci->pci_hdr.msix.next = 0;
 	/*
@@ -366,7 +371,7 @@ int virtio_pci__init(struct kvm *kvm, void *dev, struct virtio_device *vdev,
 	vpci->pci_hdr.msix.pba_offset = cpu_to_le32(1 | PCI_IO_SIZE); /* Use BAR 3 */
 	vpci->config_vector = 0;
 
-	r = irq__register_device(subsys_id, &ndev, &pin, &line);
+	r = irq__register_device(subsys_id, &pin, &line);
 	if (r < 0)
 		goto free_mmio;
 
@@ -375,7 +380,7 @@ int virtio_pci__init(struct kvm *kvm, void *dev, struct virtio_device *vdev,
 
 	vpci->pci_hdr.irq_pin	= pin;
 	vpci->pci_hdr.irq_line	= line;
-	r = pci__register(&vpci->pci_hdr, ndev);
+	r = device__register(&vpci->dev_hdr);
 	if (r < 0)
 		goto free_ioport;
 
diff --git a/x86/irq.c b/x86/irq.c
index 8dc90bf..7447c6b 100644
--- a/x86/irq.c
+++ b/x86/irq.c
@@ -17,7 +17,6 @@
 #define IRQCHIP_IOAPIC			2
 
 static u8		next_line	= 5;
-static u8		next_dev	= 1;
 static struct rb_root	pci_tree	= RB_ROOT;
 
 /* First 24 GSIs are routed between IRQCHIPs and IOAPICs */
@@ -86,7 +85,7 @@ static int insert(struct rb_root *root, struct pci_dev *data)
 	return 0;
 }
 
-int irq__register_device(u32 dev, u8 *num, u8 *pin, u8 *line)
+int irq__register_device(u32 dev, u8 *pin, u8 *line)
 {
 	struct pci_dev *node;
 	int r;
@@ -128,7 +127,6 @@ int irq__register_device(u32 dev, u8 *num, u8 *pin, u8 *line)
 		new->line	= next_line++;
 		*line		= new->line;
 		*pin		= node->pin;
-		*num		= next_dev++;
 
 		list_add(&new->node, &node->lines);