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diff --git a/Documentation/cachetlb.txt b/Documentation/cachetlb.txt new file mode 100644 index 00000000000..e132fb1163b --- /dev/null +++ b/Documentation/cachetlb.txt @@ -0,0 +1,384 @@ + 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 it's 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 vma->cpu_vm_mask), 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. + + Platform developers note that generic code will always + invoke this interface without mm->page_table_lock held. + +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. + + Platform developers note that generic code will always + invoke this interface with mm->page_table_lock held. + +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. + + Platform developers note that generic code will always + invoke this interface with mm->page_table_lock held. + +5) void flush_tlb_pgtables(struct mm_struct *mm, + unsigned long start, unsigned long end) + + The software page tables for address space 'mm' for virtual + addresses in the range 'start' to 'end-1' are being torn down. + + Some platforms cache the lowest level of the software page tables + in a linear virtually mapped array, to make TLB miss processing + more efficient. On such platforms, since the TLB is caching the + software page table structure, it needs to be flushed when parts + of the software page table tree are unlinked/freed. + + Sparc64 is one example of a platform which does this. + + Usually, when munmap()'ing an area of user virtual address + space, the kernel leaves the page table parts around and just + marks the individual pte's as invalid. However, if very large + portions of the address space are unmapped, the kernel frees up + those portions of the software page tables to prevent potential + excessive kernel memory usage caused by erratic mmap/mmunmap + sequences. It is at these times that flush_tlb_pgtables will + be invoked. + +6) void update_mmu_cache(struct vm_area_struct *vma, + unsigned long address, pte_t pte) + + At the end of every page fault, this routine is invoked to + tell the architecture specific code that a translation + described by "pte" 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. + +7) 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. + +8) void lazy_mmu_prot_update(pte_t pte) + This interface is called whenever the protection on + any user PTEs change. This interface provides a notification + to architecture specific code to take appropiate action. + + +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 + fork, exit, and exec. + +2) 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. + +3) 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. + +4) 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 + +5) 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_vm_area() 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 it's 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 it's 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_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. |