118 lines
4.6 KiB
Plaintext
118 lines
4.6 KiB
Plaintext
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Shared Memory Support
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=====================
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Prerequisites
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-------------
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These features must be enabled before shared memory support can be
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provided:
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CONFIG_ARCH_ADDRENV=y - Support for per-task address environment using a
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MMU.
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CONFIG_BUILD_KERNEL=y - Support for protected kernel-/user-space memory
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regions must be provided by the MMU.
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CONFIG_GRAN=y - The granule allocation is the allocation underlying all
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paged allocations.
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CONFIG_GRAN_SINGLE=n - Multiple granule allocators are needed: One for
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the physical page allocation and one virtual page allocator for each
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process.
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CONFIG_MM_PGALLOC=y - Enables the physical page allocator
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CONFIG_MM_PGSIZE - Determines the size of one page that can be mapped by
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the MMU.
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And then finally:
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CONFIG_MM_SHM=y - Enables shared memory support
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CONFIG_ARCH_SHM_VBASE - The virtual address of the beginning of the
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shared memory region.
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CONFIG_ARCH_SHM_MAXREGIONS - The maximum number of regions that can
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allocated for the shared memory space. This hard-coded value permits
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static allocation of the shared memory data structures and serves no
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other purpose. Default is 1.
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CONFIG_ARCH_SHM_NPAGES - The maximum number of pages that can allocated
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for the shared memory region. Default is 1.
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The size of the virtual shared memory address space is then determined by
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the product of the maximum number of regions, the maximum number of pages
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per region, and the configured size of each page.
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Concepts
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--------
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Each process has a task group structure, struct task_group_s, that holds
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information common to all threads in the group. If CONFIG_MM_SHM=y, then
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this includes data structures for the per-process shared memory virtual
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page allocator.
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A memory region is accessed using:
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int shmget(key_t key, size_t size, int shmflg);
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by a lookup using internal shared memory data sets with key as the lookup
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match value. On success, shmget returns the shared memory identifier for
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the match -- in this implementation that identifier is simply the table
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index of the match.
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If the memory region does not exist, it may also be created by shmget (if
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the IPC_CREAT bit is set in the shmflag). When a shared memory region is
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created, the following things happen:
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- A new entry is set aside in the internal data set. The key value is
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assigned to the entry and the table index is the new shared memory
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identifier.
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- The requested size is rounded up to rounded up to full pages, each of
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size CONFIG_MM_PGSIZE.
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- A set of physical pages are allocated and the physical address of
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these pages is retained in the internel data set.
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Now the key maps to and shared memory identifier (the table index) and
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the table index provides access to the list of physical pages making up
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the shared memory region.
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NOTE: An improved implementation my perform a "lazy" back up of the
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physical memory, i.e., do not allocate the physical memory until the
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memory is required, for example, when a page fault occurs when a
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application tries to allocate the memory.
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A shared memory region is destroyed via:
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int shmctl(int shmid, int cmd, struct shmid_ds *buf);
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In order for a process to make use of the memory region, it must be
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"attached" the the process using:
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FAR void *shmat(int shmid, FAR const void *shmaddr, int shmflg);
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shmat() returns the virtual address where the shared memory can be found
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in the user process. Attaching the shared memory region involves the
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following steps:
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- Use the shmid as a table index to look up the mapping in the shared
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memory internal data structures.
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- Allocate a virtual address spaces of the same size as the physical
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address space using the per-process virtual shared memory virtual
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page allocator that can be found in the calling process' task group
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structure.
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- Use platform specific interfaces to mapy the physical memory to the
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selected virtual address space, and
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- Return the allocated virtual base address to the caller.
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The memory region can be detached from the user process using:
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int shmdt(FAR const void *shmaddr);
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Relevant header files:
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---------------------
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include/sys/shm.h - Shared memory interface declarations
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include/sys/ipc.h - Provides additional definitions used by the shared
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memory interfaces
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include/nuttx/addrenv.h - Defines the virtual address space of the
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process.
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include/nuttx/pgalloc.h - Page allocator interfaces
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mm/shm/shm.h - Internal shared memory definitions. This includes the
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definitions of the internal shared memory data structures.
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