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+		Cache and TLB Flushing
+		     Under Linux
+
+	    David S. Miller <davem@redhat.com>
+
+This document describes the cache/tlb flushing interfaces called
+by the Linux VM subsystem.  It enumerates over each interface,
+describes its intended purpose, and what side effect is expected
+after the interface is invoked.
+
+The side effects described below are stated for a uniprocessor
+implementation, and what is to happen on that single processor.  The
+SMP cases are a simple extension, in that you just extend the
+definition such that the side effect for a particular interface occurs
+on all processors in the system.  Don't let this scare you into
+thinking SMP cache/tlb flushing must be so inefficient, this is in
+fact an area where many optimizations are possible.  For example,
+if it can be proven that a user address space has never executed
+on a cpu (see mm_cpumask()), one need not perform a flush
+for this address space on that cpu.
+
+First, the TLB flushing interfaces, since they are the simplest.  The
+"TLB" is abstracted under Linux as something the cpu uses to cache
+virtual-->physical address translations obtained from the software
+page tables.  Meaning that if the software page tables change, it is
+possible for stale translations to exist in this "TLB" cache.
+Therefore when software page table changes occur, the kernel will
+invoke one of the following flush methods _after_ the page table
+changes occur:
+
+1) void flush_tlb_all(void)
+
+	The most severe flush of all.  After this interface runs,
+	any previous page table modification whatsoever will be
+	visible to the cpu.
+
+	This is usually invoked when the kernel page tables are
+	changed, since such translations are "global" in nature.
+
+2) void flush_tlb_mm(struct mm_struct *mm)
+
+	This interface flushes an entire user address space from
+	the TLB.  After running, this interface must make sure that
+	any previous page table modifications for the address space
+	'mm' will be visible to the cpu.  That is, after running,
+	there will be no entries in the TLB for 'mm'.
+
+	This interface is used to handle whole address space
+	page table operations such as what happens during
+	fork, and exec.
+
+3) void flush_tlb_range(struct vm_area_struct *vma,
+			unsigned long start, unsigned long end)
+
+	Here we are flushing a specific range of (user) virtual
+	address translations from the TLB.  After running, this
+	interface must make sure that any previous page table
+	modifications for the address space 'vma->vm_mm' in the range
+	'start' to 'end-1' will be visible to the cpu.  That is, after
+	running, there will be no entries in the TLB for 'mm' for
+	virtual addresses in the range 'start' to 'end-1'.
+
+	The "vma" is the backing store being used for the region.
+	Primarily, this is used for munmap() type operations.
+
+	The interface is provided in hopes that the port can find
+	a suitably efficient method for removing multiple page
+	sized translations from the TLB, instead of having the kernel
+	call flush_tlb_page (see below) for each entry which may be
+	modified.
+
+4) void flush_tlb_page(struct vm_area_struct *vma, unsigned long addr)
+
+	This time we need to remove the PAGE_SIZE sized translation
+	from the TLB.  The 'vma' is the backing structure used by
+	Linux to keep track of mmap'd regions for a process, the
+	address space is available via vma->vm_mm.  Also, one may
+	test (vma->vm_flags & VM_EXEC) to see if this region is
+	executable (and thus could be in the 'instruction TLB' in
+	split-tlb type setups).
+
+	After running, this interface must make sure that any previous
+	page table modification for address space 'vma->vm_mm' for
+	user virtual address 'addr' will be visible to the cpu.  That
+	is, after running, there will be no entries in the TLB for
+	'vma->vm_mm' for virtual address 'addr'.
+
+	This is used primarily during fault processing.
+
+5) void update_mmu_cache(struct vm_area_struct *vma,
+			 unsigned long address, pte_t *ptep)
+
+	At the end of every page fault, this routine is invoked to
+	tell the architecture specific code that a translation
+	now exists at virtual address "address" for address space
+	"vma->vm_mm", in the software page tables.
+
+	A port may use this information in any way it so chooses.
+	For example, it could use this event to pre-load TLB
+	translations for software managed TLB configurations.
+	The sparc64 port currently does this.
+
+6) void tlb_migrate_finish(struct mm_struct *mm)
+
+	This interface is called at the end of an explicit
+	process migration. This interface provides a hook
+	to allow a platform to update TLB or context-specific
+	information for the address space.
+
+	The ia64 sn2 platform is one example of a platform
+	that uses this interface.
+
+Next, we have the cache flushing interfaces.  In general, when Linux
+is changing an existing virtual-->physical mapping to a new value,
+the sequence will be in one of the following forms:
+
+	1) flush_cache_mm(mm);
+	   change_all_page_tables_of(mm);
+	   flush_tlb_mm(mm);
+
+	2) flush_cache_range(vma, start, end);
+	   change_range_of_page_tables(mm, start, end);
+	   flush_tlb_range(vma, start, end);
+
+	3) flush_cache_page(vma, addr, pfn);
+	   set_pte(pte_pointer, new_pte_val);
+	   flush_tlb_page(vma, addr);
+
+The cache level flush will always be first, because this allows
+us to properly handle systems whose caches are strict and require
+a virtual-->physical translation to exist for a virtual address
+when that virtual address is flushed from the cache.  The HyperSparc
+cpu is one such cpu with this attribute.
+
+The cache flushing routines below need only deal with cache flushing
+to the extent that it is necessary for a particular cpu.  Mostly,
+these routines must be implemented for cpus which have virtually
+indexed caches which must be flushed when virtual-->physical
+translations are changed or removed.  So, for example, the physically
+indexed physically tagged caches of IA32 processors have no need to
+implement these interfaces since the caches are fully synchronized
+and have no dependency on translation information.
+
+Here are the routines, one by one:
+
+1) void flush_cache_mm(struct mm_struct *mm)
+
+	This interface flushes an entire user address space from
+	the caches.  That is, after running, there will be no cache
+	lines associated with 'mm'.
+
+	This interface is used to handle whole address space
+	page table operations such as what happens during exit and exec.
+
+2) void flush_cache_dup_mm(struct mm_struct *mm)
+
+	This interface flushes an entire user address space from
+	the caches.  That is, after running, there will be no cache
+	lines associated with 'mm'.
+
+	This interface is used to handle whole address space
+	page table operations such as what happens during fork.
+
+	This option is separate from flush_cache_mm to allow some
+	optimizations for VIPT caches.
+
+3) void flush_cache_range(struct vm_area_struct *vma,
+			  unsigned long start, unsigned long end)
+
+	Here we are flushing a specific range of (user) virtual
+	addresses from the cache.  After running, there will be no
+	entries in the cache for 'vma->vm_mm' for virtual addresses in
+	the range 'start' to 'end-1'.
+
+	The "vma" is the backing store being used for the region.
+	Primarily, this is used for munmap() type operations.
+
+	The interface is provided in hopes that the port can find
+	a suitably efficient method for removing multiple page
+	sized regions from the cache, instead of having the kernel
+	call flush_cache_page (see below) for each entry which may be
+	modified.
+
+4) void flush_cache_page(struct vm_area_struct *vma, unsigned long addr, unsigned long pfn)
+
+	This time we need to remove a PAGE_SIZE sized range
+	from the cache.  The 'vma' is the backing structure used by
+	Linux to keep track of mmap'd regions for a process, the
+	address space is available via vma->vm_mm.  Also, one may
+	test (vma->vm_flags & VM_EXEC) to see if this region is
+	executable (and thus could be in the 'instruction cache' in
+	"Harvard" type cache layouts).
+
+	The 'pfn' indicates the physical page frame (shift this value
+	left by PAGE_SHIFT to get the physical address) that 'addr'
+	translates to.  It is this mapping which should be removed from
+	the cache.
+
+	After running, there will be no entries in the cache for
+	'vma->vm_mm' for virtual address 'addr' which translates
+	to 'pfn'.
+
+	This is used primarily during fault processing.
+
+5) void flush_cache_kmaps(void)
+
+	This routine need only be implemented if the platform utilizes
+	highmem.  It will be called right before all of the kmaps
+	are invalidated.
+
+	After running, there will be no entries in the cache for
+	the kernel virtual address range PKMAP_ADDR(0) to
+	PKMAP_ADDR(LAST_PKMAP).
+
+	This routing should be implemented in asm/highmem.h
+
+6) void flush_cache_vmap(unsigned long start, unsigned long end)
+   void flush_cache_vunmap(unsigned long start, unsigned long end)
+
+	Here in these two interfaces we are flushing a specific range
+	of (kernel) virtual addresses from the cache.  After running,
+	there will be no entries in the cache for the kernel address
+	space for virtual addresses in the range 'start' to 'end-1'.
+
+	The first of these two routines is invoked after map_vm_area()
+	has installed the page table entries.  The second is invoked
+	before unmap_kernel_range() deletes the page table entries.
+
+There exists another whole class of cpu cache issues which currently
+require a whole different set of interfaces to handle properly.
+The biggest problem is that of virtual aliasing in the data cache
+of a processor.
+
+Is your port susceptible to virtual aliasing in its D-cache?
+Well, if your D-cache is virtually indexed, is larger in size than
+PAGE_SIZE, and does not prevent multiple cache lines for the same
+physical address from existing at once, you have this problem.
+
+If your D-cache has this problem, first define asm/shmparam.h SHMLBA
+properly, it should essentially be the size of your virtually
+addressed D-cache (or if the size is variable, the largest possible
+size).  This setting will force the SYSv IPC layer to only allow user
+processes to mmap shared memory at address which are a multiple of
+this value.
+
+NOTE: This does not fix shared mmaps, check out the sparc64 port for
+one way to solve this (in particular SPARC_FLAG_MMAPSHARED).
+
+Next, you have to solve the D-cache aliasing issue for all
+other cases.  Please keep in mind that fact that, for a given page
+mapped into some user address space, there is always at least one more
+mapping, that of the kernel in its linear mapping starting at
+PAGE_OFFSET.  So immediately, once the first user maps a given
+physical page into its address space, by implication the D-cache
+aliasing problem has the potential to exist since the kernel already
+maps this page at its virtual address.
+
+  void copy_user_page(void *to, void *from, unsigned long addr, struct page *page)
+  void clear_user_page(void *to, unsigned long addr, struct page *page)
+
+	These two routines store data in user anonymous or COW
+	pages.  It allows a port to efficiently avoid D-cache alias
+	issues between userspace and the kernel.
+
+	For example, a port may temporarily map 'from' and 'to' to
+	kernel virtual addresses during the copy.  The virtual address
+	for these two pages is chosen in such a way that the kernel
+	load/store instructions happen to virtual addresses which are
+	of the same "color" as the user mapping of the page.  Sparc64
+	for example, uses this technique.
+
+	The 'addr' parameter tells the virtual address where the
+	user will ultimately have this page mapped, and the 'page'
+	parameter gives a pointer to the struct page of the target.
+
+	If D-cache aliasing is not an issue, these two routines may
+	simply call memcpy/memset directly and do nothing more.
+
+  void flush_dcache_page(struct page *page)
+
+	Any time the kernel writes to a page cache page, _OR_
+	the kernel is about to read from a page cache page and
+	user space shared/writable mappings of this page potentially
+	exist, this routine is called.
+
+	NOTE: This routine need only be called for page cache pages
+	      which can potentially ever be mapped into the address
+	      space of a user process.  So for example, VFS layer code
+	      handling vfs symlinks in the page cache need not call
+	      this interface at all.
+
+	The phrase "kernel writes to a page cache page" means,
+	specifically, that the kernel executes store instructions
+	that dirty data in that page at the page->virtual mapping
+	of that page.  It is important to flush here to handle
+	D-cache aliasing, to make sure these kernel stores are
+	visible to user space mappings of that page.
+
+	The corollary case is just as important, if there are users
+	which have shared+writable mappings of this file, we must make
+	sure that kernel reads of these pages will see the most recent
+	stores done by the user.
+
+	If D-cache aliasing is not an issue, this routine may
+	simply be defined as a nop on that architecture.
+
+        There is a bit set aside in page->flags (PG_arch_1) as
+	"architecture private".  The kernel guarantees that,
+	for pagecache pages, it will clear this bit when such
+	a page first enters the pagecache.
+
+	This allows these interfaces to be implemented much more
+	efficiently.  It allows one to "defer" (perhaps indefinitely)
+	the actual flush if there are currently no user processes
+	mapping this page.  See sparc64's flush_dcache_page and
+	update_mmu_cache implementations for an example of how to go
+	about doing this.
+
+	The idea is, first at flush_dcache_page() time, if
+	page->mapping->i_mmap is an empty tree, just mark the architecture
+	private page flag bit.  Later, in update_mmu_cache(), a check is
+	made of this flag bit, and if set the flush is done and the flag
+	bit is cleared.
+
+	IMPORTANT NOTE: It is often important, if you defer the flush,
+			that the actual flush occurs on the same CPU
+			as did the cpu stores into the page to make it
+			dirty.  Again, see sparc64 for examples of how
+			to deal with this.
+
+  void copy_to_user_page(struct vm_area_struct *vma, struct page *page,
+                         unsigned long user_vaddr,
+                         void *dst, void *src, int len)
+  void copy_from_user_page(struct vm_area_struct *vma, struct page *page,
+                           unsigned long user_vaddr,
+                           void *dst, void *src, int len)
+	When the kernel needs to copy arbitrary data in and out
+	of arbitrary user pages (f.e. for ptrace()) it will use
+	these two routines.
+
+	Any necessary cache flushing or other coherency operations
+	that need to occur should happen here.  If the processor's
+	instruction cache does not snoop cpu stores, it is very
+	likely that you will need to flush the instruction cache
+	for copy_to_user_page().
+
+  void flush_anon_page(struct vm_area_struct *vma, struct page *page,
+                       unsigned long vmaddr)
+  	When the kernel needs to access the contents of an anonymous
+	page, it calls this function (currently only
+	get_user_pages()).  Note: flush_dcache_page() deliberately
+	doesn't work for an anonymous page.  The default
+	implementation is a nop (and should remain so for all coherent
+	architectures).  For incoherent architectures, it should flush
+	the cache of the page at vmaddr.
+
+  void flush_kernel_dcache_page(struct page *page)
+	When the kernel needs to modify a user page is has obtained
+	with kmap, it calls this function after all modifications are
+	complete (but before kunmapping it) to bring the underlying
+	page up to date.  It is assumed here that the user has no
+	incoherent cached copies (i.e. the original page was obtained
+	from a mechanism like get_user_pages()).  The default
+	implementation is a nop and should remain so on all coherent
+	architectures.  On incoherent architectures, this should flush
+	the kernel cache for page (using page_address(page)).
+
+
+  void flush_icache_range(unsigned long start, unsigned long end)
+  	When the kernel stores into addresses that it will execute
+	out of (eg when loading modules), this function is called.
+
+	If the icache does not snoop stores then this routine will need
+	to flush it.
+
+  void flush_icache_page(struct vm_area_struct *vma, struct page *page)
+	All the functionality of flush_icache_page can be implemented in
+	flush_dcache_page and update_mmu_cache. In the future, the hope
+	is to remove this interface completely.
+
+The final category of APIs is for I/O to deliberately aliased address
+ranges inside the kernel.  Such aliases are set up by use of the
+vmap/vmalloc API.  Since kernel I/O goes via physical pages, the I/O
+subsystem assumes that the user mapping and kernel offset mapping are
+the only aliases.  This isn't true for vmap aliases, so anything in
+the kernel trying to do I/O to vmap areas must manually manage
+coherency.  It must do this by flushing the vmap range before doing
+I/O and invalidating it after the I/O returns.
+
+  void flush_kernel_vmap_range(void *vaddr, int size)
+       flushes the kernel cache for a given virtual address range in
+       the vmap area.  This is to make sure that any data the kernel
+       modified in the vmap range is made visible to the physical
+       page.  The design is to make this area safe to perform I/O on.
+       Note that this API does *not* also flush the offset map alias
+       of the area.
+
+  void invalidate_kernel_vmap_range(void *vaddr, int size) invalidates
+       the cache for a given virtual address range in the vmap area
+       which prevents the processor from making the cache stale by
+       speculatively reading data while the I/O was occurring to the
+       physical pages.  This is only necessary for data reads into the
+       vmap area.