docs/devel/memory.txt - qemu - Git at Google

 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 acyclic graph of MemoryRegion objects.  Sinks
 (leaves) are RAM and MMIO regions, while other nodes represent
 buses, memory controllers, and memory regions that have been rerouted.

 In addition to MemoryRegion objects, the memory API provides AddressSpace
 objects for every root and possibly for intermediate MemoryRegions too.
 These represent memory as seen from the CPU or a device's viewpoint.

 Types of regions
 ----------------

 There are multiple types of memory regions (all represented by a single C type
 MemoryRegion):

 - RAM: a RAM region is simply a range of host memory that can be made available
   to the guest.
   You typically initialize these with memory_region_init_ram().  Some special
   purposes require the variants memory_region_init_resizeable_ram(),
   memory_region_init_ram_from_file(), or memory_region_init_ram_ptr().

 - 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.
   You initialize these with memory_region_init_io(), passing it a
   MemoryRegionOps structure describing the callbacks.

 - ROM: a ROM memory region works like RAM for reads (directly accessing
   a region of host memory), and forbids writes. You initialize these with
   memory_region_init_rom().

 - ROM device: a ROM device memory region works like RAM for reads
   (directly accessing a region of host memory), but like MMIO for
   writes (invoking a callback).  You initialize these with
   memory_region_init_rom_device().

 - IOMMU region: an IOMMU region translates addresses of accesses made to it
   and forwards them to some other target memory region.  As the name suggests,
   these are only needed for modelling an IOMMU, not for simple devices.
   You initialize these with memory_region_init_iommu().

 - 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.

   You initialize a pure container with memory_region_init().

 - 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.
   You initialize these with memory_region_init_alias().

 - reservation region: a reservation region is primarily for debugging.
   It claims I/O space that is not supposed to be handled by QEMU itself.
   The typical use is to track parts of the address space which will be
   handled by the host kernel when KVM is enabled.
   You initialize these with memory_region_init_reservation(), or by
   passing a NULL callback parameter to memory_region_init_io().

 It is valid to add subregions to a region which is not a pure container
 (that is, to an MMIO, RAM or ROM region). This means that the region
 will act like a container, except that any addresses within the container's
 region which are not claimed by any subregion are handled by the
 container itself (ie by its MMIO callbacks or RAM backing). However
 it is generally possible to achieve the same effect with a pure container
 one of whose subregions is a low priority "background" region covering
 the whole address range; this is often clearer and is preferred.
 Subregions cannot be added to an alias region.

 Migration
 ---------

 Where the memory region is backed by host memory (RAM, ROM and
 ROM device memory region types), this host memory needs to be
 copied to the destination on migration. These APIs which allocate
 the host memory for you will also register the memory so it is
 migrated:
  - memory_region_init_ram()
  - memory_region_init_rom()
  - memory_region_init_rom_device()

 For most devices and boards this is the correct thing. If you
 have a special case where you need to manage the migration of
 the backing memory yourself, you can call the functions:
  - memory_region_init_ram_nomigrate()
  - memory_region_init_rom_nomigrate()
  - memory_region_init_rom_device_nomigrate()
 which only initialize the MemoryRegion and leave handling
 migration to the caller.

 The functions:
  - memory_region_init_resizeable_ram()
  - memory_region_init_ram_from_file()
  - memory_region_init_ram_from_fd()
  - memory_region_init_ram_ptr()
  - memory_region_init_ram_device_ptr()
 are for special cases only, and so they do not automatically
 register the backing memory for migration; the caller must
 manage migration if necessary.

 Region names
 ------------

 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
 stability.

 Region lifecycle
 ----------------

 A region is created by one of the memory_region_init*() functions and
 attached to an object, which acts as its owner or parent.  QEMU ensures
 that the owner object remains alive as long as the region is visible to
 the guest, or as long as the region is in use by a virtual CPU or another
 device.  For example, the owner object will not die between an
 address_space_map operation and the corresponding address_space_unmap.

 After creation, a region can be added to an address space or a
 container with memory_region_add_subregion(), and removed using
 memory_region_del_subregion().

 Various region attributes (read-only, dirty logging, coalesced mmio,
 ioeventfd) can be changed during the region lifecycle.  They take effect
 as soon as the region is made visible.  This can be immediately, later,
 or never.

 Destruction of a memory region happens automatically when the owner
 object dies.

 If however the memory region is part of a dynamically allocated data
 structure, you should call object_unparent() to destroy the memory region
 before the data structure is freed.  For an example see VFIOMSIXInfo
 and VFIOQuirk in hw/vfio/pci.c.

 You must not destroy a memory region as long as it may be in use by a
 device or CPU.  In order to do this, as a general rule do not create or
 destroy memory regions dynamically during a device's lifetime, and only
 call object_unparent() in the memory region owner's instance_finalize
 callback.  The dynamically allocated data structure that contains the
 memory region then should obviously be freed in the instance_finalize
 callback as well.

 If you break this rule, the following situation can happen:

 - the memory region's owner had a reference taken via memory_region_ref
   (for example by address_space_map)

 - the region is unparented, and has no owner anymore

 - when address_space_unmap is called, the reference to the memory region's
   owner is leaked.


 There is an exception to the above rule: it is okay to call
 object_unparent at any time for an alias or a container region.  It is
 therefore also okay to create or destroy alias and container regions
 dynamically during a device's lifetime.

 This exceptional usage is valid because aliases and containers only help
 QEMU building the guest's memory map; they are never accessed directly.
 memory_region_ref and memory_region_unref are never called on aliases
 or containers, and the above situation then cannot happen.  Exploiting
 this exception is rarely necessary, and therefore it is discouraged,
 but nevertheless it is used in a few places.

 For regions that "have no owner" (NULL is passed at creation time), the
 machine object is actually used as the owner.  Since instance_finalize is
 never called for the machine object, you must never call object_unparent
 on regions that have no owner, unless they are aliases or containers.


 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).
 Priority values are signed, and the default value is zero. This means that
 you can use memory_region_add_subregion_overlap() both to specify a region
 that must sit 'above' any others (with a positive priority) and also a
 background region that sits 'below' others (with a negative priority).

 If the higher priority region in an overlap is a container or alias, then
 the lower priority region will appear in any "holes" that the higher priority
 region has left by not mapping subregions to that area of its address range.
 (This applies recursively -- if the subregions are themselves containers or
 aliases that leave holes then the lower priority region will appear in these
 holes too.)

 For example, suppose we have a container A of size 0x8000 with two subregions
 B and C. B is a container mapped at 0x2000, size 0x4000, priority 2; C is
 an MMIO region mapped at 0x0, size 0x6000, priority 1. B currently has two
 of its own subregions: D of size 0x1000 at offset 0 and E of size 0x1000 at
 offset 0x2000. As a diagram:

         0      1000   2000   3000   4000   5000   6000   7000   8000
         |------|------|------|------|------|------|------|------|
   A:    [                                                      ]
   C:    [CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC]
   B:                  [                          ]
   D:                  [DDDDD]
   E:                                [EEEEE]

 The regions that will be seen within this address range then are:
         [CCCCCCCCCCCC][DDDDD][CCCCC][EEEEE][CCCCC]

 Since B has higher priority than C, its subregions appear in the flat map
 even where they overlap with C. In ranges where B has not mapped anything
 C's region appears.

 If B had provided its own MMIO operations (ie it was not a pure container)
 then these would be used for any addresses in its range not handled by
 D or E, and the result would be:
         [CCCCCCCCCCCC][DDDDD][BBBBB][EEEEE][BBBBB]

 Priority values are local to a container, because the priorities of two
 regions are only compared when they are both children of the same container.
 This means that the device in charge of the container (typically modelling
 a bus or a memory controller) can use them to manage the interaction of
 its child regions without any side effects on other parts of the system.
 In the example above, the priorities of D and E are unimportant because
 they do not overlap each other. It is the relative priority of B and C
 that causes D and E to appear on top of C: D and E's priorities are never
 compared against the priority of C.

 Visibility
 ----------
 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
     discarded
   - if the subregion is a leaf (RAM or MMIO), the search terminates, returning
     this leaf region
   - 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 continued at the alias target
     (after the address is adjusted by the subregion offset and alias offset)
   - if a recursive search within a container or alias subregion does not
     find a match (because of a "hole" in the container's coverage of its
     address range), then if this is a container with its own MMIO or RAM
     backing the search terminates, returning the container itself. Otherwise
     we continue with the next subregion in priority order
 - if none of the subregions match the address then the search terminates
   with no match found

 Example memory map
 ------------------

 system_memory: container@0-2^48-1
  |
  +---- lomem: alias@0-0xdfffffff ---> #ram (0-0xdfffffff)
  |
  +---- himem: alias@0x100000000-0x11fffffff ---> #ram (0xe0000000-0xffffffff)
  |
  +---- vga-window: alias@0xa0000-0xbffff ---> #pci (0xa0000-0xbffff)
  |      (prio 1)
  |
  +---- pci-hole: alias@0xe0000000-0xffffffff ---> #pci (0xe0000000-0xffffffff)

 pci (0-2^32-1)
  |
  +--- 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

 ram: ram@0x00000000-0xffffffff

 This 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 modelled 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.

 MMIO Operations
 ---------------

 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.unaligned specifies that the *device being modelled* supports
     unaligned accesses; if false, unaligned accesses will invoke the
     appropriate bus or CPU 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 writes, if .impl.max_access_size = 1.
  - .impl.unaligned specifies that the *implementation* supports unaligned
    accesses; if false, unaligned accesses will be emulated by two aligned
    accesses.
  - .old_mmio eases the porting of code that was formerly using
    cpu_register_io_memory(). It should not be used in new code.
	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 acyclic graph of MemoryRegion objects. Sinks
	(leaves) are RAM and MMIO regions, while other nodes represent
	buses, memory controllers, and memory regions that have been rerouted.

	In addition to MemoryRegion objects, the memory API provides AddressSpace
	objects for every root and possibly for intermediate MemoryRegions too.
	These represent memory as seen from the CPU or a device's viewpoint.

	Types of regions
	----------------

	There are multiple types of memory regions (all represented by a single C type
	MemoryRegion):

	- RAM: a RAM region is simply a range of host memory that can be made available
	to the guest.
	You typically initialize these with memory_region_init_ram(). Some special
	purposes require the variants memory_region_init_resizeable_ram(),
	memory_region_init_ram_from_file(), or memory_region_init_ram_ptr().

	- 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.
	You initialize these with memory_region_init_io(), passing it a
	MemoryRegionOps structure describing the callbacks.

	- ROM: a ROM memory region works like RAM for reads (directly accessing
	a region of host memory), and forbids writes. You initialize these with
	memory_region_init_rom().

	- ROM device: a ROM device memory region works like RAM for reads
	(directly accessing a region of host memory), but like MMIO for
	writes (invoking a callback). You initialize these with
	memory_region_init_rom_device().

	- IOMMU region: an IOMMU region translates addresses of accesses made to it
	and forwards them to some other target memory region. As the name suggests,
	these are only needed for modelling an IOMMU, not for simple devices.
	You initialize these with memory_region_init_iommu().

	- 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.

	You initialize a pure container with memory_region_init().

	- 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.
	You initialize these with memory_region_init_alias().

	- reservation region: a reservation region is primarily for debugging.
	It claims I/O space that is not supposed to be handled by QEMU itself.
	The typical use is to track parts of the address space which will be
	handled by the host kernel when KVM is enabled.
	You initialize these with memory_region_init_reservation(), or by
	passing a NULL callback parameter to memory_region_init_io().

	It is valid to add subregions to a region which is not a pure container
	(that is, to an MMIO, RAM or ROM region). This means that the region
	will act like a container, except that any addresses within the container's
	region which are not claimed by any subregion are handled by the
	container itself (ie by its MMIO callbacks or RAM backing). However
	it is generally possible to achieve the same effect with a pure container
	one of whose subregions is a low priority "background" region covering
	the whole address range; this is often clearer and is preferred.
	Subregions cannot be added to an alias region.

	Migration
	---------

	Where the memory region is backed by host memory (RAM, ROM and
	ROM device memory region types), this host memory needs to be
	copied to the destination on migration. These APIs which allocate
	the host memory for you will also register the memory so it is
	migrated:
	- memory_region_init_ram()
	- memory_region_init_rom()
	- memory_region_init_rom_device()

	For most devices and boards this is the correct thing. If you
	have a special case where you need to manage the migration of
	the backing memory yourself, you can call the functions:
	- memory_region_init_ram_nomigrate()
	- memory_region_init_rom_nomigrate()
	- memory_region_init_rom_device_nomigrate()
	which only initialize the MemoryRegion and leave handling
	migration to the caller.

	The functions:
	- memory_region_init_resizeable_ram()
	- memory_region_init_ram_from_file()
	- memory_region_init_ram_from_fd()
	- memory_region_init_ram_ptr()
	- memory_region_init_ram_device_ptr()
	are for special cases only, and so they do not automatically
	register the backing memory for migration; the caller must
	manage migration if necessary.

	Region names
	------------

	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
	stability.

	Region lifecycle
	----------------

	A region is created by one of the memory_region_init*() functions and
	attached to an object, which acts as its owner or parent. QEMU ensures
	that the owner object remains alive as long as the region is visible to
	the guest, or as long as the region is in use by a virtual CPU or another
	device. For example, the owner object will not die between an
	address_space_map operation and the corresponding address_space_unmap.

	After creation, a region can be added to an address space or a
	container with memory_region_add_subregion(), and removed using
	memory_region_del_subregion().

	Various region attributes (read-only, dirty logging, coalesced mmio,
	ioeventfd) can be changed during the region lifecycle. They take effect
	as soon as the region is made visible. This can be immediately, later,
	or never.

	Destruction of a memory region happens automatically when the owner
	object dies.

	If however the memory region is part of a dynamically allocated data
	structure, you should call object_unparent() to destroy the memory region
	before the data structure is freed. For an example see VFIOMSIXInfo
	and VFIOQuirk in hw/vfio/pci.c.

	You must not destroy a memory region as long as it may be in use by a
	device or CPU. In order to do this, as a general rule do not create or
	destroy memory regions dynamically during a device's lifetime, and only
	call object_unparent() in the memory region owner's instance_finalize
	callback. The dynamically allocated data structure that contains the
	memory region then should obviously be freed in the instance_finalize
	callback as well.

	If you break this rule, the following situation can happen:

	- the memory region's owner had a reference taken via memory_region_ref
	(for example by address_space_map)

	- the region is unparented, and has no owner anymore

	- when address_space_unmap is called, the reference to the memory region's
	owner is leaked.


	There is an exception to the above rule: it is okay to call
	object_unparent at any time for an alias or a container region. It is
	therefore also okay to create or destroy alias and container regions
	dynamically during a device's lifetime.

	This exceptional usage is valid because aliases and containers only help
	QEMU building the guest's memory map; they are never accessed directly.
	memory_region_ref and memory_region_unref are never called on aliases
	or containers, and the above situation then cannot happen. Exploiting
	this exception is rarely necessary, and therefore it is discouraged,
	but nevertheless it is used in a few places.

	For regions that "have no owner" (NULL is passed at creation time), the
	machine object is actually used as the owner. Since instance_finalize is
	never called for the machine object, you must never call object_unparent
	on regions that have no owner, unless they are aliases or containers.


	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).
	Priority values are signed, and the default value is zero. This means that
	you can use memory_region_add_subregion_overlap() both to specify a region
	that must sit 'above' any others (with a positive priority) and also a
	background region that sits 'below' others (with a negative priority).

	If the higher priority region in an overlap is a container or alias, then
	the lower priority region will appear in any "holes" that the higher priority
	region has left by not mapping subregions to that area of its address range.
	(This applies recursively -- if the subregions are themselves containers or
	aliases that leave holes then the lower priority region will appear in these
	holes too.)

	For example, suppose we have a container A of size 0x8000 with two subregions
	B and C. B is a container mapped at 0x2000, size 0x4000, priority 2; C is
	an MMIO region mapped at 0x0, size 0x6000, priority 1. B currently has two
	of its own subregions: D of size 0x1000 at offset 0 and E of size 0x1000 at
	offset 0x2000. As a diagram:

	0 1000 2000 3000 4000 5000 6000 7000 8000
	\|------\|------\|------\|------\|------\|------\|------\|------\|
	A: [ ]
	C: [CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC]
	B: [ ]
	D: [DDDDD]
	E: [EEEEE]

	The regions that will be seen within this address range then are:
	[CCCCCCCCCCCC][DDDDD][CCCCC][EEEEE][CCCCC]

	Since B has higher priority than C, its subregions appear in the flat map
	even where they overlap with C. In ranges where B has not mapped anything
	C's region appears.

	If B had provided its own MMIO operations (ie it was not a pure container)
	then these would be used for any addresses in its range not handled by
	D or E, and the result would be:
	[CCCCCCCCCCCC][DDDDD][BBBBB][EEEEE][BBBBB]

	Priority values are local to a container, because the priorities of two
	regions are only compared when they are both children of the same container.
	This means that the device in charge of the container (typically modelling
	a bus or a memory controller) can use them to manage the interaction of
	its child regions without any side effects on other parts of the system.
	In the example above, the priorities of D and E are unimportant because
	they do not overlap each other. It is the relative priority of B and C
	that causes D and E to appear on top of C: D and E's priorities are never
	compared against the priority of C.

	Visibility
	----------
	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
	discarded
	- if the subregion is a leaf (RAM or MMIO), the search terminates, returning
	this leaf region
	- 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 continued at the alias target
	(after the address is adjusted by the subregion offset and alias offset)
	- if a recursive search within a container or alias subregion does not
	find a match (because of a "hole" in the container's coverage of its
	address range), then if this is a container with its own MMIO or RAM
	backing the search terminates, returning the container itself. Otherwise
	we continue with the next subregion in priority order
	- if none of the subregions match the address then the search terminates
	with no match found

	Example memory map
	------------------

	system_memory: container@0-2^48-1
	\|
	+---- lomem: alias@0-0xdfffffff ---> #ram (0-0xdfffffff)
	\|
	+---- himem: alias@0x100000000-0x11fffffff ---> #ram (0xe0000000-0xffffffff)
	\|
	+---- vga-window: alias@0xa0000-0xbffff ---> #pci (0xa0000-0xbffff)
	\| (prio 1)
	\|
	+---- pci-hole: alias@0xe0000000-0xffffffff ---> #pci (0xe0000000-0xffffffff)

	pci (0-2^32-1)
	\|
	+--- 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

	ram: ram@0x00000000-0xffffffff

	This 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 modelled 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.

	MMIO Operations
	---------------

	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.unaligned specifies that the device being modelled supports
	unaligned accesses; if false, unaligned accesses will invoke the
	appropriate bus or CPU 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 writes, if .impl.max_access_size = 1.
	- .impl.unaligned specifies that the implementation supports unaligned
	accesses; if false, unaligned accesses will be emulated by two aligned
	accesses.
	- .old_mmio eases the porting of code that was formerly using
	cpu_register_io_memory(). It should not be used in new code.