PMAP(9)                NetBSD Kernel Developer's Manual                PMAP(9)

NAME
     pmap -- machine-dependent portion of the virtual memory system

SYNOPSIS
     #include <sys/param.h>
     #include <uvm/uvm_extern.h>

     void
     pmap_init(void);

     void
     pmap_virtual_space(vaddr_t *vstartp, vaddr_t *vendp);

     vaddr_t
     pmap_steal_memory(vsize_t size, vaddr_t *vstartp, vaddr_t *vendp);

     pmap_t
     pmap_kernel(void);

     pmap_t
     pmap_create(void);

     void
     pmap_destroy(pmap_t pmap);

     void
     pmap_reference(pmap_t pmap);

     void
     pmap_fork(pmap_t src_map, pmap_t dst_map);

     long
     pmap_resident_count(pmap_t pmap);

     long
     pmap_wired_count(pmap_t pmap);

     vaddr_t
     pmap_growkernel(vaddr_t maxkvaddr);

     int
     pmap_enter(pmap_t pmap, vaddr_t va, paddr_t pa, vm_prot_t prot,
         u_int flags);

     void
     pmap_remove(pmap_t pmap, vaddr_t sva, vaddr_t eva);

     void
     pmap_remove_all(pmap_t pmap);

     void
     pmap_protect(pmap_t pmap, vaddr_t sva, vaddr_t eva, vm_prot_t prot);

     void
     pmap_unwire(pmap_t pmap, vaddr_t va);

     bool
     pmap_extract(pmap_t pmap, vaddr_t va, paddr_t *pap);

     void
     pmap_kenter_pa(vaddr_t va, paddr_t pa, vm_prot_t prot, u_int flags);

     void
     pmap_kremove(vaddr_t va, vsize_t size);

     void
     pmap_copy(pmap_t dst_map, pmap_t src_map, vaddr_t dst_addr, vsize_t len,
         vaddr_t src_addr);

     void
     pmap_update(pmap_t pmap);

     void
     pmap_activate(struct lwp *l);

     void
     pmap_deactivate(struct lwp *l);

     void
     pmap_zero_page(paddr_t pa);

     void
     pmap_copy_page(paddr_t src, paddr_t dst);

     void
     pmap_page_protect(struct vm_page *pg, vm_prot_t prot);

     bool
     pmap_clear_modify(struct vm_page *pg);

     bool
     pmap_clear_reference(struct vm_page *pg);

     bool
     pmap_is_modified(struct vm_page *pg);

     bool
     pmap_is_referenced(struct vm_page *pg);

     paddr_t
     pmap_phys_address(paddr_t cookie);

     vaddr_t
     PMAP_MAP_POOLPAGE(paddr_t pa);

     paddr_t
     PMAP_UNMAP_POOLPAGE(vaddr_t va);

     void
     PMAP_PREFER(vaddr_t hint, vaddr_t *vap, vsize_t sz, int td);

DESCRIPTION
     The pmap module is the machine-dependent portion of the NetBSD virtual
     memory system uvm(9).  The purpose of the pmap module is to manage physi-
     cal address maps, to program the memory management hardware on the sys-
     tem, and perform any cache operations necessary to ensure correct opera-
     tion of the virtual memory system.  The pmap module is also responsible
     for maintaining certain information required by uvm(9).

     In order to cope with hardware architectures that make the invalidation
     of virtual address mappings expensive (e.g., TLB invalidations, TLB
     shootdown operations for multiple processors), the pmap module is allowed
     to delay mapping invalidation or protection operations until such time as
     they are actually necessary.  The functions that are allowed to delay
     such actions are pmap_enter(), pmap_remove(), pmap_protect(),
     pmap_kenter_pa(), and pmap_kremove().  Callers of these functions must
     use the pmap_update() function to notify the pmap module that the map-
     pings need to be made correct.  Since the pmap module is provided with
     information as to which processors are using a given physical map, the
     pmap module may use whatever optimizations it has available to reduce the
     expense of virtual-to-physical mapping synchronization.

   HEADER FILES AND DATA STRUCTURES
     Machine-dependent code must provide the header file <machine/pmap.h>.
     This file contains the definition of the pmap structure:

           struct pmap {
                   /* Contents defined by pmap implementation. */
           };
           typedef struct pmap *pmap_t;

     This header file may also define other data structures that the pmap
     implementation uses.

     Note that all prototypes for pmap interface functions are provided by the
     header file <uvm/uvm_pmap.h>.  It is possible to override this behavior
     by defining the C pre-processor macro PMAP_EXCLUDE_DECLS.  This may be
     used to add a layer of indirection to pmap API calls, for handling dif-
     ferent MMU types in a single pmap module, for example.  If the
     PMAP_EXCLUDE_DECLS macro is defined, <machine/pmap.h> must provide func-
     tion prototypes in a block like so:

           #ifdef _KERNEL /* not exposed to user namespace */
           __BEGIN_DECLS  /* make safe for C++ */
           /* Prototypes go here. */
           __END_DECLS
           #endif /* _KERNEL */

     The header file <uvm/uvm_pmap.h> defines a structure for tracking pmap
     statistics (see below).  This structure is defined as:

           struct pmap_statistics {
                   long        resident_count; /* number of mapped pages */
                   long        wired_count;    /* number of wired pages */
           };

   WIRED MAPPINGS
     The pmap module is based on the premise that all information contained in
     the physical maps it manages is redundant.  That is, physical map infor-
     mation may be ``forgotten'' by the pmap module in the event that it is
     necessary to do so; it can be rebuilt by uvm(9) by taking a page fault.
     There is one exception to this rule: so-called ``wired'' mappings may not
     be forgotten.  Wired mappings are those for which either no high-level
     information exists with which to rebuild the mapping, or mappings which
     are needed by critical sections of code where taking a page fault is
     unacceptable.  Information about which mappings are wired is provided to
     the pmap module when a mapping is established.

   MODIFIED/REFERENCED INFORMATION
     The pmap module is required to keep track of whether or not a page man-
     aged by the virtual memory system has been referenced or modified.  This
     information is used by uvm(9) to determine what happens to the page when
     scanned by the pagedaemon.

     Many CPUs provide hardware support for tracking modified/referenced
     information.  However, many CPUs, particularly modern RISC CPUs, do not.
     On CPUs which lack hardware support for modified/referenced tracking, the
     pmap module must emulate it in software.  There are several strategies
     for doing this, and the best strategy depends on the CPU.

     The ``referenced'' attribute is used by the pagedaemon to determine if a
     page is ``active''.  Active pages are not candidates for re-use in the
     page replacement algorithm.  Accurate referenced information is not
     required for correct operation; if supplying referenced information for a
     page is not feasible, then the pmap implementation should always consider
     the ``referenced'' attribute to be false.

     The ``modified'' attribute is used by the pagedaemon to determine if a
     page needs to be cleaned (written to backing store; swap space, a regular
     file, etc.).  Accurate modified information must be provided by the pmap
     module for correct operation of the virtual memory system.

     Note that modified/referenced information is only tracked for pages man-
     aged by the virtual memory system (i.e., pages for which a vm_page struc-
     ture exists).  In addition, only ``managed'' mappings of those pages have
     modified/referenced tracking.  Mappings entered with the pmap_enter()
     function are ``managed'' mappings.  It is possible for ``unmanaged'' map-
     pings of a page to be created, using the pmap_kenter_pa() function.  The
     use of ``unmanaged'' mappings should be limited to code which may execute
     in interrupt context (for example, the kernel memory allocator), or to
     enter mappings for physical addresses which are not managed by the vir-
     tual memory system.  ``Unmanaged'' mappings may only be entered into the
     kernel's virtual address space.  This constraint is placed on the callers
     of the pmap_kenter_pa() and pmap_kremove() functions so that the pmap
     implementation need not block interrupts when manipulating data struc-
     tures or holding locks.

     Also note that the modified/referenced information must be tracked on a
     per-page basis; they are not attributes of a mapping, but attributes of a
     page.  Therefore, even after all mappings for a given page have been
     removed, the modified/referenced information for that page must be pre-
     served.  The only time the modified/referenced attributes may be cleared
     is when the virtual memory system explicitly calls the
     pmap_clear_modify() and pmap_clear_reference() functions.  These func-
     tions must also change any internal state necessary to detect the page
     being modified or referenced again after the modified or referenced state
     is cleared.  (Prior to NetBSD 1.6, pmap implementations could get away
     without this because UVM (and Mach VM before that) always called
     pmap_page_protect() before clearing the modified or referenced state, but
     UVM has been changed to not do this anymore, so all pmap implementations
     must now handle this.)

   STATISTICS
     The pmap is required to keep statistics as to the number of ``resident''
     pages and the number of ``wired'' pages.

     A ``resident'' page is one for which a mapping exists.  This statistic is
     used to compute the resident size of a process and enforce resource lim-
     its.  Only pages (whether managed by the virtual memory system or not)
     which are mapped into a physical map should be counted in the resident
     count.

     A ``wired'' page is one for which a wired mapping exists.  This statistic
     is used to enforce resource limits.

     Note that it is recommended (though not required) that the pmap implemen-
     tation use the pmap_statistics structure in the tracking of pmap statis-
     tics by placing it inside the pmap structure and adjusting the counts
     when mappings are established, changed, or removed.  This avoids poten-
     tially expensive data structure traversals when the statistics are
     queried.

   REQUIRED FUNCTIONS
     This section describes functions that a pmap module must provide to the
     virtual memory system.

           void pmap_init(void)
                   This function initializes the pmap module.  It is called by
                   uvm_init() to initialize any data structures that the mod-
                   ule needs to manage physical maps.

           pmap_t pmap_kernel(void)
                   A machine independent macro which expands to
                   kernel_pmap_ptr.  This variable must be exported by the
                   platform's pmap module and it must point to the kernel
                   pmap.

           void pmap_virtual_space(vaddr_t *vstartp, vaddr_t *vendp)
                   The pmap_virtual_space() function is called to determine
                   the initial kernel virtual address space beginning and end.
                   These values are used to create the kernel's virtual memory
                   map.  The function must set *vstartp to the first kernel
                   virtual address that will be managed by uvm(9), and must
                   set *vendp to the last kernel virtual address that will be
                   managed by uvm(9).

                   If the pmap_growkernel() feature is used by a pmap imple-
                   mentation, then *vendp should be set to the maximum kernel
                   virtual address allowed by the implementation.  If
                   pmap_growkernel() is not used, then *vendp must be set to
                   the maximum kernel virtual address that can be mapped with
                   the resources currently allocated to map the kernel virtual
                   address space.

           pmap_t pmap_create(void)
                   Create a physical map and return it to the caller.  The
                   reference count on the new map is 1.

           void pmap_destroy(pmap_t pmap)
                   Drop the reference count on the specified physical map.  If
                   the reference count drops to 0, all resources associated
                   with the physical map are released and the physical map
                   destroyed.  In the case of a drop-to-0, no mappings will
                   exist in the map.  The pmap implementation may assert this.

           void pmap_reference(pmap_t pmap)
                   Increment the reference count on the specified physical
                   map.

           long pmap_resident_count(pmap_t pmap)
                   Query the ``resident pages'' statistic for pmap.

                   Note that this function may be provided as a C pre-proces-
                   sor macro.

           long pmap_wired_count(pmap_t pmap)
                   Query the ``wired pages'' statistic for pmap.

                   Note that this function may be provided as a C pre-proces-
                   sor macro.

           int pmap_enter(pmap_t pmap, vaddr_t va, paddr_t pa, vm_prot_t prot,
                   u_int flags)
                   Create a mapping in physical map pmap for the physical
                   address pa at the virtual address va with protection speci-
                   fied by bits in prot:

                         VM_PROT_READ       The mapping must allow reading.

                         VM_PROT_WRITE      The mapping must allow writing.

                         VM_PROT_EXECUTE    The page mapped contains instruc-
                                            tions that will be executed by the
                                            processor.

                   The flags argument contains protection bits (the same bits
                   as used in the prot argument) indicating the type of access
                   that caused the mapping to be created.  This information
                   may be used to seed modified/referenced information for the
                   page being mapped, possibly avoiding redundant faults on
                   platforms that track modified/referenced information in
                   software.  Other information provided by flags:

                         PMAP_WIRED      The mapping being created is a wired
                                         mapping.

                         PMAP_CANFAIL    The call to pmap_enter() is allowed
                                         to fail.  If this flag is not set,
                                         and the pmap_enter() call is unable
                                         to create the mapping, perhaps due to
                                         insufficient resources, the pmap mod-
                                         ule must panic.

                         PMAP_NOCACHE    The mapping being created is not
                                         cached.  Write accesses have a write-
                                         through policy.  No speculative mem-
                                         ory accesses.

                         PMAP_WRITE_COMBINE
                                         The mapping being created is not
                                         cached.  Writes are combined and done
                                         in one burst.  Speculative read
                                         accesses may be allowed.

                         PMAP_WRITE_BACK
                                         All accesses to the created mapping
                                         are cached.  On reads, cachelines
                                         become shared or exclusive if allo-
                                         cated on cache miss.  On writes,
                                         cachelines become modified on a cache
                                         miss.

                         PMAP_NOCACHE_OVR
                                         Same as PMAP_NOCACHE but mapping is
                                         overrideable (e.g. on x86 by MTRRs).

                   The access type provided in the flags argument will never
                   exceed the protection specified by prot.  The pmap imple-
                   mentation may assert this.  Note that on systems that do
                   not provide hardware support for tracking modified/refer-
                   enced information, modified/referenced information for the
                   page must be seeded with the access type provided in flags
                   if the PMAP_WIRED flag is set.  This is to prevent a fault
                   for the purpose of tracking modified/referenced information
                   from occurring while the system is in a critical section
                   where a fault would be unacceptable.

                   Note that pmap_enter() is sometimes called to enter a map-
                   ping at a virtual address for which a mapping already
                   exists.  In this situation, the implementation must take
                   whatever action is necessary to invalidate the previous
                   mapping before entering the new one.

                   Also note that pmap_enter() is sometimes called to change
                   the protection for a pre-existing mapping, or to change the
                   ``wired'' attribute for a pre-existing mapping.

                   The pmap_enter() function returns 0 on success or an error
                   code indicating the mode of failure.

           void pmap_remove(pmap_t pmap, vaddr_t sva, vaddr_t eva)
                   Remove mappings from the virtual address range sva to eva
                   from the specified physical map.

           void pmap_remove_all(pmap_t pmap)
                   This function is a hint to the pmap implementation that all
                   entries in pmap will be removed before any more entries are
                   entered.  Following this call, there will be pmap_remove()
                   calls resulting in every mapping being removed, followed by
                   either pmap_destroy() or pmap_update().  No other pmap
                   interfaces which take pmap as an argument will be called
                   during this process.  Other interfaces which might need to
                   access pmap (such as pmap_page_protect()) are permitted
                   during this process.

                   The pmap implementation is free to either remove all the
                   pmap's mappings immediately in pmap_remove_all(), or to use
                   the knowledge of the upcoming pmap_remove() calls to opti-
                   mize the removals (or to just ignore this call).

           void pmap_protect(pmap_t pmap, vaddr_t sva, vaddr_t eva, vm_prot_t
                   prot)
                   Set the protection of the mappings in the virtual address
                   range sva to eva in the specified physical map.

           void pmap_unwire(pmap_t pmap, vaddr_t va)
                   Clear the ``wired'' attribute on the mapping for virtual
                   address va.

           bool pmap_extract(pmap_t pmap, vaddr_t va, paddr_t *pap)
                   This function extracts a mapping from the specified physi-
                   cal map.  It serves two purposes: to determine if a mapping
                   exists for the specified virtual address, and to determine
                   what physical address is mapped at the specified virtual
                   address.  The pmap_extract() should return the physical
                   address for any kernel-accessible address, including KSEG-
                   style direct-mapped kernel addresses.

                   The pmap_extract() function returns false if a mapping for
                   va does not exist.  Otherwise, it returns true and places
                   the physical address mapped at va into *pap if the pap
                   argument is non-NULL.

           void pmap_kenter_pa(vaddr_t va, paddr_t pa, vm_prot_t prot, u_int
                   flags)
                   Enter an ``unmanaged'' mapping for physical address pa at
                   virtual address va with protection specified by bits in
                   prot:

                         VM_PROT_READ       The mapping must allow reading.

                         VM_PROT_WRITE      The mapping must allow writing.

                         VM_PROT_EXECUTE    The page mapped contains instruc-
                                            tions that will be executed by the
                                            processor.

                   Information provided by flags:

                         PMAP_NOCACHE    The mapping being created is not
                                         cached.  Write accesses have a write-
                                         through policy.  No speculative mem-
                                         ory accesses.

                         PMAP_WRITE_COMBINE
                                         The mapping being created is not
                                         cached.  Writes are combined and done
                                         in one burst.  Speculative read
                                         accesses may be allowed.

                         PMAP_WRITE_BACK
                                         All accesses to the created mapping
                                         are cached.  On reads, cachelines
                                         become shared or exclusive if allo-
                                         cated on cache miss.  On writes,
                                         cachelines become modified on a cache
                                         miss.

                         PMAP_NOCACHE_OVR
                                         Same as PMAP_NOCACHE but mapping is
                                         overrideable (e.g. on x86 by MTRRs).

                   Mappings of this type are always ``wired'', and are unaf-
                   fected by routines that alter the protection of pages (such
                   as pmap_page_protect()).  Such mappings are also not
                   included in the gathering of modified/referenced informa-
                   tion about a page.  Mappings entered with pmap_kenter_pa()
                   by machine-independent code must not have execute permis-
                   sion, as the data structures required to track execute per-
                   mission of a page may not be available to pmap_kenter_pa().
                   Machine-independent code is not allowed to enter a mapping
                   with pmap_kenter_pa() at a virtual address for which a
                   valid mapping already exists.  Mappings created with
                   pmap_kenter_pa() may be removed only with a call to
                   pmap_kremove().

                   Note that pmap_kenter_pa() must be safe for use in inter-
                   rupt context.  splvm() blocks interrupts that might cause
                   pmap_kenter_pa() to be called.

           void pmap_kremove(vaddr_t va, vsize_t size)
                   Remove all mappings starting at virtual address va for size
                   bytes from the kernel physical map.  All mappings that are
                   removed must be the ``unmanaged'' type created with
                   pmap_kenter_pa().  The implementation may assert this.

           void pmap_copy(pmap_t dst_map, pmap_t src_map, vaddr_t dst_addr,
                   vsize_t len, vaddr_t src_addr)
                   This function copies the mappings starting at src_addr in
                   src_map for len bytes into dst_map starting at dst_addr.

                   Note that while this function is required to be provided by
                   a pmap implementation, it is not actually required to do
                   anything.  pmap_copy() is merely advisory (it is used in
                   the fork(2) path to ``pre-fault'' the child's address
                   space).

           void pmap_update(pmap_t pmap)
                   This function is used to inform the pmap module that all
                   physical mappings, for the specified pmap, must now be cor-
                   rect.  That is, all delayed virtual-to-physical mappings
                   updates (such as TLB invalidation or address space identi-
                   fier updates) must be completed.  This routine must be used
                   after calls to pmap_enter(), pmap_remove(), pmap_protect(),
                   pmap_kenter_pa(), and pmap_kremove() in order to ensure
                   correct operation of the virtual memory system.

                   If a pmap implementation does not delay virtual-to-physical
                   mapping updates, pmap_update() has no operation.  In this
                   case, the call may be deleted using a C pre-processor macro
                   in <machine/pmap.h>.

           void pmap_activate(struct lwp *l)
                   Activate the physical map used by the process behind lwp l.
                   This is called by the virtual memory system when the vir-
                   tual memory context for a process is changed, and is also
                   often used by machine-dependent context switch code to pro-
                   gram the memory management hardware with the process's page
                   table base, etc.  Note that pmap_activate() may not always
                   be called when l is the current lwp.  pmap_activate() must
                   be able to handle this scenario.

           void pmap_deactivate(struct lwp *l)
                   Deactivate the physical map used by the process behind lwp
                   l.  It is generally used in conjunction with
                   pmap_activate().  Like pmap_activate(), pmap_deactivate()
                   may not always be called when l is the current lwp.

           void pmap_zero_page(paddr_t pa)
                   Zero the PAGE_SIZE sized region starting at physical
                   address pa.  The pmap implementation must take whatever
                   steps are necessary to map the page to a kernel-accessible
                   address and zero the page.  It is suggested that implemen-
                   tations use an optimized zeroing algorithm, as the perfor-
                   mance of this function directly impacts page fault perfor-
                   mance.  The implementation may assume that the region is
                   PAGE_SIZE aligned and exactly PAGE_SIZE bytes in length.

                   Note that the cache configuration of the platform should
                   also be considered in the implementation of
                   pmap_zero_page().  For example, on systems with a physi-
                   cally-addressed cache, the cache load caused by zeroing the
                   page will not be wasted, as the zeroing is usually done on-
                   demand.  However, on systems with a virtually-addressed
                   cached, the cache load caused by zeroing the page will be
                   wasted, as the page will be mapped at a virtual address
                   which is different from that used to zero the page.  In the
                   virtually-addressed cache case, care should also be taken
                   to avoid cache alias problems.

           void pmap_copy_page(paddr_t src, paddr_t dst)
                   Copy the PAGE_SIZE sized region starting at physical
                   address src to the same sized region starting at physical
                   address dst.  The pmap implementation must take whatever
                   steps are necessary to map the source and destination pages
                   to a kernel-accessible address and perform the copy.  It is
                   suggested that implementations use an optimized copy algo-
                   rithm, as the performance of this function directly impacts
                   page fault performance.  The implementation may assume that
                   both regions are PAGE_SIZE aligned and exactly PAGE_SIZE
                   bytes in length.

                   The same cache considerations that apply to
                   pmap_zero_page() apply to pmap_copy_page().

           void pmap_page_protect(struct vm_page *pg, vm_prot_t prot)
                   Lower the permissions for all mappings of the page pg to
                   prot.  This function is used by the virtual memory system
                   to implement copy-on-write (called with VM_PROT_READ set in
                   prot) and to revoke all mappings when cleaning a page
                   (called with no bits set in prot).  Access permissions must
                   never be added to a page as a result of this call.

           bool pmap_clear_modify(struct vm_page *pg)
                   Clear the ``modified'' attribute on the page pg.

                   The pmap_clear_modify() function returns true or false
                   indicating whether or not the ``modified'' attribute was
                   set on the page before it was cleared.

                   Note that this function may be provided as a C pre-proces-
                   sor macro.

           bool pmap_clear_reference(struct vm_page *pg)
                   Clear the ``referenced'' attribute on the page pg.

                   The pmap_clear_reference() function returns true or false
                   indicating whether or not the ``referenced'' attribute was
                   set on the page before it was cleared.

                   Note that this function may be provided as a C pre-proces-
                   sor macro.

           bool pmap_is_modified(struct vm_page *pg)
                   Test whether or not the ``modified'' attribute is set on
                   page pg.

                   Note that this function may be provided as a C pre-proces-
                   sor macro.

           bool pmap_is_referenced(struct vm_page *pg)
                   Test whether or not the ``referenced'' attribute is set on
                   page pg.

                   Note that this function may be provided as a C pre-proces-
                   sor macro.

           paddr_t pmap_phys_address(paddr_t cookie)
                   Convert a cookie returned by a device mmap() function into
                   a physical address.  This function is provided to accommo-
                   date systems which have physical address spaces larger than
                   can be directly addressed by the platform's paddr_t type.
                   The existence of this function is highly dubious, and it is
                   expected that this function will be removed from the pmap
                   API in a future release of NetBSD.

                   Note that this function may be provided as a C pre-proces-
                   sor macro.

   OPTIONAL FUNCTIONS
     This section describes several optional functions in the pmap API.

           vaddr_t pmap_steal_memory(vsize_t size, vaddr_t *vstartp, vaddr_t
                   *vendp)
                   This function is a bootstrap memory allocator, which may be
                   provided as an alternative to the bootstrap memory alloca-
                   tor used within uvm(9) itself.  It is particularly useful
                   on systems which provide for example a direct-mapped memory
                   segment.  This function works by stealing pages from the
                   (to be) managed memory pool, which has already been pro-
                   vided to uvm(9) in the vm_physmem[] array.  The pages are
                   then mapped, or otherwise made accessible to the kernel, in
                   a machine-dependent way.  The memory must be zeroed by
                   pmap_steal_memory().  Note that memory allocated with
                   pmap_steal_memory() will never be freed, and mappings made
                   by pmap_steal_memory() must never be ``forgotten''.

                   Note that pmap_steal_memory() should not be used as a gen-
                   eral-purpose early-startup memory allocation routine.  It
                   is intended to be used only by the uvm_pageboot_alloc()
                   routine and its supporting routines.  If you need to allo-
                   cate memory before the virtual memory system is initial-
                   ized, use uvm_pageboot_alloc().  See uvm(9) for more infor-
                   mation.

                   The pmap_steal_memory() function returns the kernel-acces-
                   sible address of the allocated memory.  If no memory can be
                   allocated, or if allocated memory cannot be mapped, the
                   function must panic.

                   If the pmap_steal_memory() function uses address space from
                   the range provided to uvm(9) by the pmap_virtual_space()
                   call, then pmap_steal_memory() must adjust *vstartp and
                   *vendp upon return.

                   The pmap_steal_memory() function is enabled by defining the
                   C pre-processor macro PMAP_STEAL_MEMORY in
                   <machine/pmap.h>.

           vaddr_t pmap_growkernel(vaddr_t maxkvaddr)
                   Management of the kernel virtual address space is compli-
                   cated by the fact that it is not always safe to wait for
                   resources with which to map a kernel virtual address.  How-
                   ever, it is not always desirable to pre-allocate all
                   resources necessary to map the entire kernel virtual
                   address space.

                   The pmap_growkernel() interface is designed to help allevi-
                   ate this problem.  The virtual memory startup code may
                   choose to allocate an initial set of mapping resources
                   (e.g., page tables) and set an internal variable indicating
                   how much kernel virtual address space can be mapped using
                   those initial resources.  Then, when the virtual memory
                   system wishes to map something at an address beyond that
                   initial limit, it calls pmap_growkernel() to pre-allocate
                   more sources with which to create the mapping.  Note that
                   once additional kernel virtual address space mapping
                   resources have been allocated, they should not be freed; it
                   is likely they will be needed again.

                   The pmap_growkernel() function returns the new maximum ker-
                   nel virtual address that can be mapped with the resources
                   it has available.  If new resources cannot be allocated,
                   pmap_growkernel() must panic.

                   The pmap_growkernel() function is enabled by defining the C
                   pre-processor macro PMAP_GROWKERNEL in <machine/pmap.h>.

           void pmap_fork(pmap_t src_map, pmap_t dst_map)
                   Some pmap implementations may need to keep track of other
                   information not directly related to the virtual address
                   space.  For example, on the i386 port, the Local Descriptor
                   Table state of a process is associated with the pmap (this
                   is due to the fact that applications manipulate the Local
                   Descriptor Table directly expect it to be logically associ-
                   ated with the virtual memory state of the process).

                   The pmap_fork() function is provided as a way to associate
                   information from src_map with dst_map when a vmspace is
                   forked.  pmap_fork() is called from uvmspace_fork().

                   The pmap_fork() function is enabled by defining the C pre-
                   processor macro PMAP_FORK in <machine/pmap.h>.

           vaddr_t PMAP_MAP_POOLPAGE(paddr_t pa)
                   This function is used by the pool(9) memory pool manager.
                   Pools allocate backing pages one at a time.  This is pro-
                   vided as a means to use hardware features such as a direct-
                   mapped memory segment to map the pages used by the pool(9)
                   allocator.  This can lead to better performance by e.g.
                   reducing TLB contention.

                   PMAP_MAP_POOLPAGE() returns the kernel-accessible address
                   of the page being mapped.  It must always succeed.

                   The use of PMAP_MAP_POOLPAGE() is enabled by defining it as
                   a C pre-processor macro in <machine/pmap.h>.  If
                   PMAP_MAP_POOLPAGE() is defined, PMAP_UNMAP_POOLPAGE() must
                   also be defined.

                   The following is an example of how to define
                   PMAP_MAP_POOLPAGE():

                         #define PMAP_MAP_POOLPAGE(pa)   MIPS_PHYS_TO_KSEG0((pa))

                   This takes the physical address of a page and returns the
                   KSEG0 address of that page on a MIPS processor.

           paddr_t PMAP_UNMAP_POOLPAGE(vaddr_t va)
                   This function is the inverse of PMAP_MAP_POOLPAGE().

                   PMAP_UNMAP_POOLPAGE() returns the physical address of the
                   page corresponding to the provided kernel-accessible
                   address.

                   The use of PMAP_UNMAP_POOLPAGE() is enabled by defining it
                   as a C pre-processor macro in <machine/pmap.h>.  If
                   PMAP_UNMAP_POOLPAGE() is defined, PMAP_MAP_POOLPAGE() must
                   also be defined.

                   The following is an example of how to define
                   PMAP_UNMAP_POOLPAGE():

                         #define PMAP_UNMAP_POOLPAGE(pa) MIPS_KSEG0_TO_PHYS((va))

                   This takes the KSEG0 address of a previously-mapped pool
                   page and returns the physical address of that page on a
                   MIPS processor.

           void PMAP_PREFER(vaddr_t hint, vaddr_t *vap, vsize_t sz, int td)
                   This function is used by uvm_map(9) to adjust a virtual
                   address being allocated in order to avoid cache alias prob-
                   lems.  If necessary, the virtual address pointed by vap
                   will be advanced.  hint is an object offset which will be
                   mapped into the resulting virtual address, and sz is size
                   of the object.  td indicates if the machine dependent pmap
                   uses the topdown VM.

                   The use of PMAP_PREFER() is enabled by defining it as a C
                   pre-processor macro in <machine/pmap.h>.

           void pmap_procwr(struct proc *p, vaddr_t va, vsize_t size)
                   Synchronize CPU instruction caches of the specified range.
                   The address space is designated by p.  This function is
                   typically used to flush instruction caches after code modi-
                   fication.

                   The use of pmap_procwr() is enabled by defining a C pre-
                   processor macro PMAP_NEED_PROCWR in <machine/pmap.h>.

SEE ALSO
     uvm(9)

HISTORY
     The pmap module was originally part of the design of the virtual memory
     system in the Mach Operating System.  The goal was to provide a clean
     separation between the machine-independent and the machine-dependent por-
     tions of the virtual memory system, in stark contrast to the original
     3BSD virtual memory system, which was specific to the VAX.

     Between 4.3BSD and 4.4BSD, the Mach virtual memory system, including the
     pmap API, was ported to BSD and included in the 4.4BSD release.

     NetBSD inherited the BSD version of the Mach virtual memory system.
     NetBSD 1.4 was the first NetBSD release with the new uvm(9) virtual mem-
     ory system, which included several changes to the pmap API.  Since the
     introduction of uvm(9), the pmap API has evolved further.

AUTHORS
     The original Mach VAX pmap module was written by Avadis Tevanian, Jr. and
     Michael Wayne Young.

     Mike Hibler did the integration of the Mach virtual memory system into
     4.4BSD and implemented a pmap module for the Motorola
     68020+68851/68030/68040.

     The pmap API as it exists in NetBSD is derived from 4.4BSD, and has been
     modified by
     Chuck Cranor,
     Charles M. Hannum,
     Chuck Silvers,
     Wolfgang Solfrank,
     Bill Sommerfeld, and
     Jason R. Thorpe.

     The author of this document is
     Jason R. Thorpe <thorpej@NetBSD.org>.

BUGS
     The use and definition of pmap_activate() and pmap_deactivate() needs to
     be reexamined.

     The use of pmap_copy() needs to be reexamined.  Empirical evidence sug-
     gests that performance of the system suffers when pmap_copy() actually
     performs its defined function.  This is largely due to the fact that the
     copy of the virtual-to-physical mappings is wasted if the process calls
     execve(2) after fork(2).  For this reason, it is recommended that pmap
     implementations leave the body of the pmap_copy() function empty for now.

NetBSD 6.0                     November 4, 2009                     NetBSD 6.0

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