docs/memory: Explictly state that MemoryRegion priority is signed

[qemu.git] / docs / memory.txt

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 four 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.

- 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.


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 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
guest.

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).

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
  - 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
------------------

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

Attributes
----------

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 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.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 writes, if .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.