Shared Memory Support
=====================

Prerequisites
-------------
  These features must be enabled before shared memory support can be
  provided:

    CONFIG_ARCH_ADDRENV=y - Support for per-task address environment using a
      MMU.
    CONFIG_BUILD_KERNEL=y - Support for protected kernel-/user-space memory
      regions must be provided by the MMU.
    CONFIG_GRAN=y - The granule allocation is the allocation underlying all
      paged allocations.
    CONFIG_MM_PGALLOC=y - Enables the physical page allocator
    CONFIG_MM_PGSIZE - Determines the size of one page that can be mapped by
      the MMU.

  And then finally:

    CONFIG_MM_SHM=y - Enables shared memory support
    CONFIG_ARCH_SHM_VBASE - The virtual address of the beginning of the
      shared memory region.
    CONFIG_ARCH_SHM_MAXREGIONS - The maximum number of regions that can
      allocated for the shared memory space.  This hard-coded value permits
      static allocation of the shared memory data structures and serves no
      other purpose. Default is 1.
    CONFIG_ARCH_SHM_NPAGES - The maximum number of pages that can allocated
      for the shared memory region.  Default is 1.

  The size of the virtual shared memory address space is then determined by
  the product of the maximum number of regions, the maximum number of pages
  per region, and the configured size of each page.

Concepts
--------
  Each process has a task group structure, struct task_group_s, that holds
  information common to all threads in the group.  If CONFIG_MM_SHM=y, then
  this includes data structures for the per-process shared memory virtual
  page allocator.

  A memory region is accessed using:

    int shmget(key_t key, size_t size, int shmflg);

  by a lookup using internal shared memory data sets with key as the lookup
  match value.  On success, shmget returns the shared memory identifier for
  the match -- in this implementation that identifier is simply the table
  index of the match.

  If the memory region does not exist, it may also be created by shmget (if
  the IPC_CREAT bit is set in the shmflag).  When a shared memory region is
  created, the following things happen:

    - A new entry is set aside in the internal data set.  The key value is
      assigned to the entry and the table index is the new shared memory
      identifier.

    - The requested size is rounded up to rounded up to full pages, each of
      size CONFIG_MM_PGSIZE.

    - A set of physical pages are allocated and the physical address of
      these pages is retained in the internel data set.

  Now the key maps to and shared memory identifier (the table index) and
  the table index provides access to the list of physical pages making up
  the shared memory region.

  NOTE: An improved implementation my perform a "lazy" back up of the
  physical memory, i.e., do not allocate the physical memory until the
  memory is required, for example, when a page fault occurs when a
  application tries to allocate the memory.

  A shared memory region is destroyed via:

    int shmctl(int shmid, int cmd, struct shmid_ds *buf);

  In order for a process to make use of the memory region, it must be
  "attached" the process using:

    FAR void *shmat(int shmid, FAR const void *shmaddr, int shmflg);

  shmat() returns the virtual address where the shared memory can be found
  in the user process.  Attaching the shared memory region involves the
  following steps:

    - Use the shmid as a table index to look up the mapping in the shared
      memory internal data structures.

    - Allocate a virtual address spaces of the same size as the physical
      address space using the per-process virtual shared memory virtual
      page allocator that can be found in the calling process' task group
      structure.

    - Use platform specific interfaces to mapy the physical memory to the
      selected virtual address space, and

    - Return the allocated virtual base address to the caller.

  The memory region can be detached from the user process using:

    int shmdt(FAR const void *shmaddr);

Relevant header files:
---------------------

  include/sys/shm.h - Shared memory interface declarations
  include/sys/ipc.h - Provides additional definitions used by the shared
    memory interfaces
  include/nuttx/addrenv.h - Defines the virtual address space of the
    process.
  include/nuttx/pgalloc.h - Page allocator interfaces
  mm/shm/shm.h - Internal shared memory definitions.  This includes the
    definitions of the internal shared memory data structures.