diff --git a/pci.c b/pci.c
index 95df169..59b2618 100644
--- a/pci.c
+++ b/pci.c
@@ -11,8 +11,12 @@ 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 */
-static u32 io_space_blocks		= KVM_32BIT_GAP_START + 0x1000000;
+/* This is within our PCI gap - in an unused area.
+ * Note this is a PCI *bus address*, is used to assign BARs etc.!
+ * (That's why it can still 32bit even with 64bit guests-- 64bit
+ * PCI isn't currently supported.)
+ */
+static u32 io_space_blocks		= KVM_PCI_MMIO_AREA;
 
 u32 pci_get_io_space_block(u32 size)
 {
diff --git a/x86/include/kvm/kvm-arch.h b/x86/include/kvm/kvm-arch.h
index 02aa8b9..686b1b8 100644
--- a/x86/include/kvm/kvm-arch.h
+++ b/x86/include/kvm/kvm-arch.h
@@ -18,6 +18,11 @@
 
 #define KVM_MMIO_START		KVM_32BIT_GAP_START
 
+/* This is the address that pci_get_io_space_block() starts allocating
+ * from.  Note that this is a PCI bus address (though same on x86).
+ */
+#define KVM_PCI_MMIO_AREA	(KVM_MMIO_START + 0x1000000)
+
 struct kvm {
 	int			sys_fd;		/* For system ioctls(), i.e. /dev/kvm */
 	int			vm_fd;		/* For VM ioctls() */