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PMAP_INIT(9) FreeBSD Kernel Developer's Manual PMAP_INIT(9) NAME pmap_init, pmap_enter, pmap_kenter_pa, pmap_remove, pmap_kremove, pmap_unwire, pmap_protect, pmap_page_protect, pmap_is_modified, pmap_clear_modify, pmap_is_referenced, pmap_clear_reference, pmap_copy_page, pmap_zero_page, pmap_create, pmap_reference, pmap_destroy, pmap_steal_memory, pmap_growkernel, pmap_update, pmap_collect, pmap_populate, pmap_virtual_space - machine dependent interface to the MMU SYNOPSIS #include <machine/pmap.h> DESCRIPTION The architecture-dependent pmap module describes how the physical mapping is done between the user-processes and kernel virtual addresses and the physical addresses of the main memory, providing machine-dependent translation and access tables that are used directly or indirectly by the memory-management hardware. The pmap layer can be viewed as a big array of mapping entries that are indexed by virtual address to produce a physical address and flags. These flags describe the page's protection, whether the page has been referenced or modified and other characteristics. The pmap interface is consistent across all platforms and hides the way page mappings are stored. INITIALIZATION void pmap_init(void); The pmap_init() function is called from the machine-independent uvm_init(9) initialization code, when the MMU is enabled. PAGE MANAGEMENT Modified/referenced information is only tracked for pages managed by UVM (pages for which a vm_page structure exists). Only managed mappings of those pages have modified/referenced tracking. The use of unmanaged mappings should be limited to code which may execute in interrupt context (such as malloc(9)) or to enter mappings for physical addresses which are not managed by UVM. This allows pmap modules to avoid blocking interrupts when manipulating data structures or holding locks. Unmanaged mappings may only be entered into the kernel's virtual address space. The modified/referenced bits must be tracked on a per-page basis, as 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 bits for that page must be preserved. The only time the modified/referenced bits may be cleared is when UVM explicitly calls the pmap_clear_modify() and pmap_clear_reference() functions. These functions must also change any internal state necessary to detect the page being modified or referenced again after the modified/referenced state is cleared. Mappings entered by pmap_enter() are managed, mappings entered by pmap_kenter_pa() are not. MAPPING ALLOCATION int pmap_enter(pmap_t pmap, vaddr_t va, paddr_t pa, vm_prot_t prot, int flags); void pmap_kenter_pa(vaddr_t va, paddr_t pa, vm_prot_t prot); void pmap_remove(pmap_t pmap, vaddr_t sva, vaddr_t eva); void pmap_kremove(vaddr_t va, vsize_t size); The pmap_enter() function creates a managed mapping for physical page pa at the specified virtual address va in the target physical map pmap with protection specified by prot: PROT_READ The mapping must allow reading. PROT_WRITE The mapping must allow writing. PROT_EXEC The page mapped contains instructions that will be executed by the processor. The flags argument contains protection bits (the same bits 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 module must panic. The access type provided in the flags argument will never exceed the protection specified by prot. The pmap_enter() function is called by the fault routine to establish a mapping for the page being faulted in. If pmap_enter() is called to enter a mapping at a virtual address for which a mapping already exists, the previous mapping must be invalidated. 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_kenter_pa() function creates an unmanaged mapping of physical address pa at the specified virtual address va with the protection specified by prot. The pmap_remove() function removes the range of virtual addresses sva to eva from pmap, assuming proper alignment. pmap_remove() is called during an unmap operation to remove low-level machine dependent mappings. The pmap_kremove() function removes an unmanaged mapping at virtual address va of size size. A call to pmap_update() must be made after pmap_kenter_pa() or pmap_kremove() to notify the pmap layer that the mappings need to be made correct. ACCESS PROTECTION void pmap_unwire(pmap_t pmap, vaddr_t va); void pmap_protect(pmap_t pmap, vaddr_t sva, vaddr_t eva, vm_prot_t prot); void pmap_page_protect(struct vm_page *pg, vm_prot_t prot); The pmap_unwire() function clears the wired attribute for a map/virtual- address pair. The mapping must already exist in pmap. The pmap_protect() function sets the physical protection on range sva to eva, in pmap. The pmap_protect() function is called during a copy-on-write operation to write protect copy-on-write memory, and when paging out a page to remove all mappings of a page. The pmap_page_protect() function sets the permission for all mapping to page pg. The pmap_page_protect() function is called before a pageout operation to ensure that all pmap references to a page are removed. PHYSICAL PAGE-USAGE INFORMATION boolean_t pmap_is_modified(struct vm_page *pg); boolean_t pmap_clear_modify(struct vm_page *pg); boolean_t pmap_is_referenced(struct vm_page *pg); boolean_t pmap_clear_reference(struct vm_page *pg); The pmap_is_modified() and pmap_clear_modify() functions read/set the modify bits on the specified physical page pg. The pmap_is_referenced() and pmap_clear_reference() functions read/set the reference bits on the specified physical page pg. The pmap_is_referenced() and pmap_is_modified() functions are called by the pagedaemon when looking for pages to free. The pmap_clear_referenced() and pmap_clear_modify() functions are called by the pagedaemon to help identification of pages that are no longer in demand. PHYSICAL PAGE INITIALIZATION void pmap_copy_page(struct vm_page *src, struct vm_page *dst); void pmap_zero_page(struct vm_page *page); The pmap_copy_page() function copies the content of the physical page src to physical page dst. The pmap_zero_page() function fills page with zeroes. INTERNAL DATA STRUCTURE MANAGEMENT pmap_t pmap_create(void); void pmap_reference(pmap_t pmap); void pmap_destroy(pmap_t pmap); The pmap_create() function creates an instance of the pmap structure. The pmap_reference() function increments the reference count on pmap. The pmap_destroy() function decrements the reference count on physical map pmap and retires it from service if the count drops to zero, assuming it contains no valid mappings. OPTIONAL FUNCTIONS vaddr_t pmap_steal_memory(vsize_t size, vaddr_t *vstartp, vaddr_t *vendp); vaddr_t pmap_growkernel(vaddr_t maxkvaddr); void pmap_update(pmap_t pmap); void pmap_collect(pmap_t pmap); void pmap_populate(pmap_t pmap, vaddr_t va); void pmap_virtual_space(vaddr_t *vstartp, vaddr_t *vendp); Wired memory allocation before the virtual memory system is bootstrapped is accomplished by the pmap_steal_memory() function. After that point, the kernel memory allocation routines should be used. The pmap_growkernel() function can preallocate kernel page tables to a specified virtual address. The pmap_update() function notifies the pmap module to force processing of all delayed actions for all pmaps. The pmap_populate() function makes sure the resources necessary for mapping the specified virtual address va are allocated in the target physical map pmap. The pmap_collect() function informs the pmap module that the given pmap is not expected to be used for some time, giving the pmap module a chance to prioritize. The initial bounds of the kernel virtual address space are returned by pmap_virtual_space(). SEE ALSO fork(2), uvm_init(9) HISTORY The 4.4BSD pmap module is based on Mach 3.0. The introduction of UVM left the pmap interface unchanged for the most part. BUGS Ifdefs must be documented. pmap_update() should be mandatory. FreeBSD 14.1-RELEASE-p8 January 19, 2025 FreeBSD 14.1-RELEASE-p8