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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, here 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 and ->i_mmap_nonlinear
- an empty list, 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 2.7 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.