VxWorks Reference Manual : Libraries
mmuSh7700Lib - Hitachi SH7700 MMU support library
mmuSh7700LibInit( ) - initialize module
mmuLib.c provides the architecture dependent routines that directly control the memory management unit. It provides 10 routines that are called by the higher level architecture independent routines in vmLib.c:
mmuLibInit - initialize module mmuTransTblCreate - create a new translation table mmuTransTblDelete - delete a translation table. mmuEnable - turn mmu on or off mmuStateSet - set state of virtual memory page mmuStateGet - get state of virtual memory page mmuPageMap - map physical memory page to virtual memory page mmuGlobalPageMap - map physical memory page to global virtual memory page mmuTranslate - translate a virtual address to a physical address mmuCurrentSet - change active translation table
Applications using the mmu will never call these routines directly; the visable interface is supported in vmLib.c.
mmuLib supports the creation and maintenance of multiple translation tables, one of which is the active translation table when the mmu is enabled. Note that VxWorks does not include a translation table as part of the task context; individual tasks do not reside in private virtual memory. However, we include the facilities to create multiple translation tables so that the user may create "private" virtual memory contexts and switch them in an application specific manner. New translation tables are created with a call to mmuTransTblCreate, and installed as the active translation table with mmuCurrentSet. Translation tables are modified and potentially augmented with calls to mmuPageMap and mmuStateSet. The state of portions of the translation table can be read with calls to mmuStateGet and mmuTranslate.
The traditional VxWorks architecture and design philosophy requires that all objects and operating systems resources be visable and accessable to all agents (tasks, isrs, watchdog timers, etc) in the system. This has traditionally been insured by the fact that all objects and data structures reside in physical memory; thus, a data structure created by one agent may be accessed by any other agent using the same pointer (object identifiers in VxWorks are often pointers to data structures.) This creates a potential problem if you have multiple virtual memory contexts. For example, if a semaphore is created in one virtual memory context, you must gurantee that that semaphore will be visable in all virtual memory contexts if the semaphore is to be accessed at interrupt level, when a virtual memory context other than the one in which it was created may be active. Another example is that code loaded using the incremental loader from the shell must be accessable in all virtual memory contexts, since code is shared by all agents in the system.
This problem is resolved by maintaining a global "transparent" mapping of virtual to physical memory for all the contiguous segments of physical memory (on board memory, i/o space, sections of vme space, etc) that is shared by all translation tables; all available physical memory appears at the same address in virtual memory in all virtual memory contexts. This technique provides an environment that allows resources that rely on a globally accessable physical address to run without modification in a system with multiple virtual memory contexts.
An additional requirement is that modifications made to the state of global virtual memory in one translation table appear in all translation tables. For example, memory containing the text segment is made read only (to avoid accidental corruption) by setting the appropriate writeable bits in the translation table entries corresponding to the virtual memory containing the text segment. This state information must be shared by all virtual memory contexts, so that no matter what translation table is active, the text segment is protected from corruption. The mechanism that implements this feature is architecture dependent, but usually entails building a section of a translation table that corresponds to the global memory, that is shared by all other translation tables. Thus, when changes to the state of the global memory are made in one translation table, the changes are reflected in all other translation tables.
mmuLib provides a seperate call for constructing global virtual memory - mmuGlobalPageMap - which creates translation table entries that are shared by all translation tables. Initialization code in usrConfig makes calls to vmGlobalMap (which in turn calls mmuGlobalPageMap) to set up global transparent virtual memory for all available physical memory. All calls made to mmuGlobaPageMap must occur before any virtual memory contexts are created; changes made to global virtual memory after virtual memory contexts are created are not guaranteed to be reflected in all virtual memory contexts.
Most mmu architectures will dedicate some fixed amount of virtual memory to a minimal section of the translation table (a "segment", or "block"). This creates a problem in that the user may map a small section of virtual memory into the global translation tables, and then attempt to use the virtual memory after this section as private virtual memory. The problem is that the translation table entries for this virtual memory are contained in the global translation tables, and are thus shared by all translation tables. This condition is detected by vmMap, and an error is returned, thus, the lower level routines in mmuLib.c (mmuPageMap, mmuGlobalPageMap) need not perform any error checking.
A global variable called mmuPageBlockSize should be defined which is equal to the minimum virtual segment size.
This module supports the SH7700 mmu with a two level translation table:
root | | -------------------------------- top level | td | td | td | td | td | td | ... -------------------------------- | | | | | | | | | | | | ---------- | v v v v | ----- NULL NULL NULL NULL | | v v ------ ------ l | ptel | | ptel | o ------ ------ w | ptel | | ptel | e ------ ------ r | ptel | | ptel | l ------ ------ e | ptel | | ptel | v ------ ------ e . . l . . . .where the top level consists of an array of pointers (Table Descriptors) held within a single 4k page. These point to arrays of PTEL (Page Table Entry Low) arrays in the lower level. Each of these lower level arrays is also held within a single 4k page, and describes a virtual space of 4MB (each page descriptor is 4 bytes, so we get 1024 of these in each array, and each page descriptor maps a 4KB page - thus 1024 * 4096 = 4MB.)To implement global virtual memory, a seperate translation table called mmuGlobalTransTbl is created when the module is initialized. Calls to mmuGlobalPageMap will augment and modify this translation table. When new translation tables are created, memory for the top level array of td's is allocated and initialized by duplicating the pointers in mmuGlobalTransTbl's top level td array. Thus, the new translation table will use the global translation table's state information for portions of virtual memory that are defined as global. Here's a picture to illustrate:
GLOBAL TRANS TBL NEW TRANS TBL root root | | | | ------------------------- ------------------------- top level | td1 | td2 | NULL| NULL| | td1 | td2 | NULL| NULL| ------------------------- ------------------------- | | | | | | | | | | | | | | | | ---------- | v v ---------- | v v | ------ NULL NULL | | NULL NULL | | | | o------------------------------------ | | | | | o----------------------------------------- | | v v ------ ------ l | ptel | | ptel | o ------ ------ w | ptel | | ptel | e ------ ------ r | ptel | | ptel | l ------ ------ e | ptel | | ptel | v ------ ------ e . . l . . . .Note that with this scheme, the global memory granularity is 4MB. Each time you map a section of global virtual memory, you dedicate at least 4MB of the virtual space to global virtual memory that will be shared by all virtual memory contexts.The physcial memory that holds these data structures is obtained from the system memory manager via memalign to insure that the memory is page aligned. We want to protect this memory from being corrupted, so we invalidate the descriptors that we set up in the global translation that correspond to the memory containing the translation table data structures. This creates a "chicken and the egg" paradox, in that the only way we can modify these data structures is through virtual memory that is now invalidated, and we can't validate it because the page descriptors for that memory are in invalidated memory (confused yet?) So, you will notice that anywhere that page table descriptors (ptel's) are modified, we do so by locking out interrupts, momentarily disabling the mmu, accessing the memory with its physical address, enabling the mmu, and then re-enabling interrupts (see mmuStateSet, for example.)
1) Memory fragmentation - mmuLib obtains memory from the system memory
manager via memalign to contain the mmu's translation tables. This memory
was allocated a page at a time on page boundries. Unfortunately, in the
current memory management scheme, the memory manager is not able to allocate
these pages contiguously. Building large translation tables (ie, when
mapping large portions of virtual memory) causes excessive fragmentation
of the system memory pool. An attempt to alleviate this has been installed
by providing a local buffer of page aligned memory; the user may control
the buffer size by manipulating the global variable mmuNumPagesInFreeList.
By default, mmuPagesInFreeList is set to 8.2) Alternate memory source - A customer has special purpose hardware that
includes seperate static RAM for the mmu's translation tables. Thus, they
require the ability to specify an alternate source of memory other than
memalign. A global variable has been created that points to the memory
partition to be used as the source for translation table memory; by default,
it points to the system memory partition. The user may modify this to
point to another memory partition before mmuSh7700LibInit is called.
mmuSh7700LibInit( ) - initialize module
STATUS mmuSh7700LibInit ( int pageSize )
Build a dummy translation table that will hold the page table entries for the global translation table. The mmu remains disabled upon completion. Note that this routine is global so that it may be referenced in usrConfig.c to pull in the correct mmuLib for the specific architecture.
OK or ERROR