new file mode 100644
@@ -0,0 +1,172 @@
+The memory API
+The memory API models the memory and I/O buses and controllers of a QEMU
+machine. It attempts to allow modelling of:
+ - ordinary RAM
+ - memory-mapped I/O (MMIO)
+ - memory controllers that can dynamically reroute physical memory regions
+ to different destinations
+The memory model provides support for
+ - tracking RAM changes by the guest
+ - setting up coalesced memory for kvm
+ - setting up ioeventfd regions for kvm
+Memory is modelled as an tree (really acyclic graph) of MemoryRegion objects.
+The root of the tree is memory as seen from the CPU's viewpoint (the system
+bus). Nodes in the tree represent other buses, memory controllers, and
+memory regions that have been rerouted. Leaves are RAM and MMIO regions.
+Types of regions
+There are four types of memory regions (all represented by a single C type
+- RAM: a RAM region is simply a range of host memory that can be made available
+ to the guest.
+- MMIO: a range of guest memory that is implemented by host callbacks;
+ each read or write causes a callback to be called on the host.
+- container: a container simply includes other memory regions, each at
+ a different offset. Containers are useful for grouping several regions
+ into one unit. For example, a PCI BAR may be composed of a RAM region
+ and an MMIO region.
+ A container's subregions are usually non-overlapping. In some cases it is
+ useful to have overlapping regions; for example a memory controller that
+ can overlay a subregion of RAM with MMIO or ROM, or a PCI controller
+ that does not prevent card from claiming overlapping BARs.
+- alias: a subsection of another region. Aliases allow a region to be
+ split apart into discontiguous regions. Examples of uses are memory banks
+ used when the guest address space is smaller than the amount of RAM
+ addressed, or a memory controller that splits main memory to expose a "PCI
+ hole". Aliases may point to any type of region, including other aliases,
+ but an alias may not point back to itself, directly or indirectly.
+Regions are assigned names by the constructor. For most regions these are
+only used for debugging purposes, but RAM regions also use the name to identify
+live migration sections. This means that RAM region names need to have ABI
+A region is created by one of the constructor functions (memory_region_init*())
+and destroyed by the destructor (memory_region_destroy()). In between,
+a region can be added to an address space by using memory_region_add_subregion()
+and removed using memory_region_del_subregion(). Region attributes may be
+changed at any point; they take effect once the region becomes exposed to the
+Overlapping regions and priority
+Usually, regions may not overlap each other; a memory address decodes into
+exactly one target. In some cases it is useful to allow regions to overlap,
+and sometimes to control which of an overlapping regions is visible to the
+guest. This is done with memory_region_add_subregion_overlap(), which
+allows the region to overlap any other region in the same container, and
+specifies a priority that allows the core to decide which of two regions at
+the same address are visible (highest wins).
+The memory core uses the following rules to select a memory region when the
+guest accesses an address:
+- all direct subregions of the root region are matched against the address, in
+ descending priority order
+ - if the address lies outside the region offset/size, the subregion is
+ - if the subregion is a leaf (RAM or MMIO), the seach terminates
+ - if the subregion is a container, the same algorithm is used within the
+ subregion (after the address is adjusted by the subregion offset)
+ - if the subregion is an alias, the search is continues at the alias target
+ (after the address is adjusted by the subregion offset and alias offset)
+Example memory map
+ +---- lomem: alias@0-0xdfffffff ---> #ram (0-0xdfffffff)
+ +---- himem: alias@0x100000000-0x11fffffff ---> #ram (0xe0000000-0xffffffff)
+ +---- vga-window: alias@0xa0000-0xbfffff ---> #pci (0xa0000-0xbffff)
+ | (prio 1)
+ +---- pci-hole: alias@0xe0000000-0xffffffff ---> #pci (0xe0000000-0xffffffff)
+ +--- vga-area: container@0xa0000-0xbffff
+ | |
+ | +--- alias@0x00000-0x7fff ---> #vram (0x010000-0x017fff)
+ | |
+ | +--- alias@0x08000-0xffff ---> #vram (0x020000-0x027fff)
+ +---- vram: ram@0xe1000000-0xe1ffffff
+ +---- vga-mmio: mmio@0xe2000000-0xe200ffff
+The is a (simplified) PC memory map. The 4GB RAM block is mapped into the
+system address space via two aliases: "lomem" is a 1:1 mapping of the first
+3.5GB; "himem" maps the last 0.5GB at address 4GB. This leaves 0.5GB for the
+so-called PCI hole, that allows a 32-bit PCI bus to exist in a system with
+4GB of memory.
+The memory controller diverts addresses in the range 640K-768K to the PCI
+address space. This is modeled using the "vga-window" alias, mapped at a
+higher priority so it obscures the RAM at the same addresses. The vga window
+can be removed by programming the memory controller; this is modelled by
+removing the alias and exposing the RAM underneath.
+The pci address space is not a direct child of the system address space, since
+we only want parts of it to be visible (we accomplish this using aliases).
+It has two subregions: vga-area models the legacy vga window and is occupied
+by two 32K memory banks pointing at two sections of the framebuffer.
+In addition the vram is mapped as a BAR at address e1000000, and an additional
+BAR containing MMIO registers is mapped after it.
+Note that if the guest maps a BAR outside the PCI hole, it would not be
+visible as the pci-hole alias clips it to a 0.5GB range.
+Various region attributes (read-only, dirty logging, coalesced mmio, ioeventfd)
+can be changed during the region lifecycle. They take effect once the region
+is made visible (which can be immediately, later, or never).
+MMIO regions are provided with ->read() and ->write() callbacks; in addition
+various constraints can be supplied to control how these callbacks are called:
+ - .valid.min_access_size, .valid.max_access_size define the access sizes
+ (in bytes) which the device accepts; accesses outside this range will
+ have device and bus specific behaviour (ignored, or machine check)
+ - .valid.aligned specifies that the device only accepts naturally aligned
+ accesses. Unaligned accesses invoke device and bus specific behaviour.
+ - .impl.min_access_size, .impl.max_access_size define the access sizes
+ (in bytes) supported by the *implementation*; other access sizes will be
+ emulated using the ones available. For example a 4-byte write will be
+ emulated using four 1-byte write, is .impl.max_access_size = 1.
+ - .impl.valid specifies that the *implementation* only supports unaligned
+ accesses; unaligned accesses will be emulated by two aligned accesses.
+ - .old_portio and .old_mmio can be used to ease porting from code using
+ cpu_register_io_memory() and register_ioport(). They should not be used
+ in new code.