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docs/memory.txt: Improve list of different memory regions

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Improve the part of the memory region documentation which describes
the various different kinds of memory region:
 * add the missing types ROM, IOMMU and reservation
 * mention the functions used to initialize each type, as a hint
   for finding the API docs and examples of use

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <1454007297-3971-1-git-send-email-peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Peter Maydell 2016-01-28 18:54:57 +00:00 committed by Paolo Bonzini
parent 37eebb8693
commit 5056c0c3de
1 changed files with 25 additions and 1 deletions
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docs/memory.txt
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@ -26,14 +26,28 @@ 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
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_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), 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
@ -45,12 +59,22 @@ MemoryRegion):
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