Archipelago » qemu

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

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).

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 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

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

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

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.

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 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.