Based on kernel version 4.8. Page generated on 2016-10-06 23:10 EST.
1 Cache and TLB Flushing 2 Under Linux 3 4 David S. Miller <firstname.lastname@example.org> 5 6 This document describes the cache/tlb flushing interfaces called 7 by the Linux VM subsystem. It enumerates over each interface, 8 describes its intended purpose, and what side effect is expected 9 after the interface is invoked. 10 11 The side effects described below are stated for a uniprocessor 12 implementation, and what is to happen on that single processor. The 13 SMP cases are a simple extension, in that you just extend the 14 definition such that the side effect for a particular interface occurs 15 on all processors in the system. Don't let this scare you into 16 thinking SMP cache/tlb flushing must be so inefficient, this is in 17 fact an area where many optimizations are possible. For example, 18 if it can be proven that a user address space has never executed 19 on a cpu (see mm_cpumask()), one need not perform a flush 20 for this address space on that cpu. 21 22 First, the TLB flushing interfaces, since they are the simplest. The 23 "TLB" is abstracted under Linux as something the cpu uses to cache 24 virtual-->physical address translations obtained from the software 25 page tables. Meaning that if the software page tables change, it is 26 possible for stale translations to exist in this "TLB" cache. 27 Therefore when software page table changes occur, the kernel will 28 invoke one of the following flush methods _after_ the page table 29 changes occur: 30 31 1) void flush_tlb_all(void) 32 33 The most severe flush of all. After this interface runs, 34 any previous page table modification whatsoever will be 35 visible to the cpu. 36 37 This is usually invoked when the kernel page tables are 38 changed, since such translations are "global" in nature. 39 40 2) void flush_tlb_mm(struct mm_struct *mm) 41 42 This interface flushes an entire user address space from 43 the TLB. After running, this interface must make sure that 44 any previous page table modifications for the address space 45 'mm' will be visible to the cpu. That is, after running, 46 there will be no entries in the TLB for 'mm'. 47 48 This interface is used to handle whole address space 49 page table operations such as what happens during 50 fork, and exec. 51 52 3) void flush_tlb_range(struct vm_area_struct *vma, 53 unsigned long start, unsigned long end) 54 55 Here we are flushing a specific range of (user) virtual 56 address translations from the TLB. After running, this 57 interface must make sure that any previous page table 58 modifications for the address space 'vma->vm_mm' in the range 59 'start' to 'end-1' will be visible to the cpu. That is, after 60 running, there will be no entries in the TLB for 'mm' for 61 virtual addresses in the range 'start' to 'end-1'. 62 63 The "vma" is the backing store being used for the region. 64 Primarily, this is used for munmap() type operations. 65 66 The interface is provided in hopes that the port can find 67 a suitably efficient method for removing multiple page 68 sized translations from the TLB, instead of having the kernel 69 call flush_tlb_page (see below) for each entry which may be 70 modified. 71 72 4) void flush_tlb_page(struct vm_area_struct *vma, unsigned long addr) 73 74 This time we need to remove the PAGE_SIZE sized translation 75 from the TLB. The 'vma' is the backing structure used by 76 Linux to keep track of mmap'd regions for a process, the 77 address space is available via vma->vm_mm. Also, one may 78 test (vma->vm_flags & VM_EXEC) to see if this region is 79 executable (and thus could be in the 'instruction TLB' in 80 split-tlb type setups). 81 82 After running, this interface must make sure that any previous 83 page table modification for address space 'vma->vm_mm' for 84 user virtual address 'addr' will be visible to the cpu. That 85 is, after running, there will be no entries in the TLB for 86 'vma->vm_mm' for virtual address 'addr'. 87 88 This is used primarily during fault processing. 89 90 5) void update_mmu_cache(struct vm_area_struct *vma, 91 unsigned long address, pte_t *ptep) 92 93 At the end of every page fault, this routine is invoked to 94 tell the architecture specific code that a translation 95 now exists at virtual address "address" for address space 96 "vma->vm_mm", in the software page tables. 97 98 A port may use this information in any way it so chooses. 99 For example, it could use this event to pre-load TLB 100 translations for software managed TLB configurations. 101 The sparc64 port currently does this. 102 103 6) void tlb_migrate_finish(struct mm_struct *mm) 104 105 This interface is called at the end of an explicit 106 process migration. This interface provides a hook 107 to allow a platform to update TLB or context-specific 108 information for the address space. 109 110 The ia64 sn2 platform is one example of a platform 111 that uses this interface. 112 113 Next, we have the cache flushing interfaces. In general, when Linux 114 is changing an existing virtual-->physical mapping to a new value, 115 the sequence will be in one of the following forms: 116 117 1) flush_cache_mm(mm); 118 change_all_page_tables_of(mm); 119 flush_tlb_mm(mm); 120 121 2) flush_cache_range(vma, start, end); 122 change_range_of_page_tables(mm, start, end); 123 flush_tlb_range(vma, start, end); 124 125 3) flush_cache_page(vma, addr, pfn); 126 set_pte(pte_pointer, new_pte_val); 127 flush_tlb_page(vma, addr); 128 129 The cache level flush will always be first, because this allows 130 us to properly handle systems whose caches are strict and require 131 a virtual-->physical translation to exist for a virtual address 132 when that virtual address is flushed from the cache. The HyperSparc 133 cpu is one such cpu with this attribute. 134 135 The cache flushing routines below need only deal with cache flushing 136 to the extent that it is necessary for a particular cpu. Mostly, 137 these routines must be implemented for cpus which have virtually 138 indexed caches which must be flushed when virtual-->physical 139 translations are changed or removed. So, for example, the physically 140 indexed physically tagged caches of IA32 processors have no need to 141 implement these interfaces since the caches are fully synchronized 142 and have no dependency on translation information. 143 144 Here are the routines, one by one: 145 146 1) void flush_cache_mm(struct mm_struct *mm) 147 148 This interface flushes an entire user address space from 149 the caches. That is, after running, there will be no cache 150 lines associated with 'mm'. 151 152 This interface is used to handle whole address space 153 page table operations such as what happens during exit and exec. 154 155 2) void flush_cache_dup_mm(struct mm_struct *mm) 156 157 This interface flushes an entire user address space from 158 the caches. That is, after running, there will be no cache 159 lines associated with 'mm'. 160 161 This interface is used to handle whole address space 162 page table operations such as what happens during fork. 163 164 This option is separate from flush_cache_mm to allow some 165 optimizations for VIPT caches. 166 167 3) void flush_cache_range(struct vm_area_struct *vma, 168 unsigned long start, unsigned long end) 169 170 Here we are flushing a specific range of (user) virtual 171 addresses from the cache. After running, there will be no 172 entries in the cache for 'vma->vm_mm' for virtual addresses in 173 the range 'start' to 'end-1'. 174 175 The "vma" is the backing store being used for the region. 176 Primarily, this is used for munmap() type operations. 177 178 The interface is provided in hopes that the port can find 179 a suitably efficient method for removing multiple page 180 sized regions from the cache, instead of having the kernel 181 call flush_cache_page (see below) for each entry which may be 182 modified. 183 184 4) void flush_cache_page(struct vm_area_struct *vma, unsigned long addr, unsigned long pfn) 185 186 This time we need to remove a PAGE_SIZE sized range 187 from the cache. The 'vma' is the backing structure used by 188 Linux to keep track of mmap'd regions for a process, the 189 address space is available via vma->vm_mm. Also, one may 190 test (vma->vm_flags & VM_EXEC) to see if this region is 191 executable (and thus could be in the 'instruction cache' in 192 "Harvard" type cache layouts). 193 194 The 'pfn' indicates the physical page frame (shift this value 195 left by PAGE_SHIFT to get the physical address) that 'addr' 196 translates to. It is this mapping which should be removed from 197 the cache. 198 199 After running, there will be no entries in the cache for 200 'vma->vm_mm' for virtual address 'addr' which translates 201 to 'pfn'. 202 203 This is used primarily during fault processing. 204 205 5) void flush_cache_kmaps(void) 206 207 This routine need only be implemented if the platform utilizes 208 highmem. It will be called right before all of the kmaps 209 are invalidated. 210 211 After running, there will be no entries in the cache for 212 the kernel virtual address range PKMAP_ADDR(0) to 213 PKMAP_ADDR(LAST_PKMAP). 214 215 This routing should be implemented in asm/highmem.h 216 217 6) void flush_cache_vmap(unsigned long start, unsigned long end) 218 void flush_cache_vunmap(unsigned long start, unsigned long end) 219 220 Here in these two interfaces we are flushing a specific range 221 of (kernel) virtual addresses from the cache. After running, 222 there will be no entries in the cache for the kernel address 223 space for virtual addresses in the range 'start' to 'end-1'. 224 225 The first of these two routines is invoked after map_vm_area() 226 has installed the page table entries. The second is invoked 227 before unmap_kernel_range() deletes the page table entries. 228 229 There exists another whole class of cpu cache issues which currently 230 require a whole different set of interfaces to handle properly. 231 The biggest problem is that of virtual aliasing in the data cache 232 of a processor. 233 234 Is your port susceptible to virtual aliasing in its D-cache? 235 Well, if your D-cache is virtually indexed, is larger in size than 236 PAGE_SIZE, and does not prevent multiple cache lines for the same 237 physical address from existing at once, you have this problem. 238 239 If your D-cache has this problem, first define asm/shmparam.h SHMLBA 240 properly, it should essentially be the size of your virtually 241 addressed D-cache (or if the size is variable, the largest possible 242 size). This setting will force the SYSv IPC layer to only allow user 243 processes to mmap shared memory at address which are a multiple of 244 this value. 245 246 NOTE: This does not fix shared mmaps, check out the sparc64 port for 247 one way to solve this (in particular SPARC_FLAG_MMAPSHARED). 248 249 Next, you have to solve the D-cache aliasing issue for all 250 other cases. Please keep in mind that fact that, for a given page 251 mapped into some user address space, there is always at least one more 252 mapping, that of the kernel in its linear mapping starting at 253 PAGE_OFFSET. So immediately, once the first user maps a given 254 physical page into its address space, by implication the D-cache 255 aliasing problem has the potential to exist since the kernel already 256 maps this page at its virtual address. 257 258 void copy_user_page(void *to, void *from, unsigned long addr, struct page *page) 259 void clear_user_page(void *to, unsigned long addr, struct page *page) 260 261 These two routines store data in user anonymous or COW 262 pages. It allows a port to efficiently avoid D-cache alias 263 issues between userspace and the kernel. 264 265 For example, a port may temporarily map 'from' and 'to' to 266 kernel virtual addresses during the copy. The virtual address 267 for these two pages is chosen in such a way that the kernel 268 load/store instructions happen to virtual addresses which are 269 of the same "color" as the user mapping of the page. Sparc64 270 for example, uses this technique. 271 272 The 'addr' parameter tells the virtual address where the 273 user will ultimately have this page mapped, and the 'page' 274 parameter gives a pointer to the struct page of the target. 275 276 If D-cache aliasing is not an issue, these two routines may 277 simply call memcpy/memset directly and do nothing more. 278 279 void flush_dcache_page(struct page *page) 280 281 Any time the kernel writes to a page cache page, _OR_ 282 the kernel is about to read from a page cache page and 283 user space shared/writable mappings of this page potentially 284 exist, this routine is called. 285 286 NOTE: This routine need only be called for page cache pages 287 which can potentially ever be mapped into the address 288 space of a user process. So for example, VFS layer code 289 handling vfs symlinks in the page cache need not call 290 this interface at all. 291 292 The phrase "kernel writes to a page cache page" means, 293 specifically, that the kernel executes store instructions 294 that dirty data in that page at the page->virtual mapping 295 of that page. It is important to flush here to handle 296 D-cache aliasing, to make sure these kernel stores are 297 visible to user space mappings of that page. 298 299 The corollary case is just as important, if there are users 300 which have shared+writable mappings of this file, we must make 301 sure that kernel reads of these pages will see the most recent 302 stores done by the user. 303 304 If D-cache aliasing is not an issue, this routine may 305 simply be defined as a nop on that architecture. 306 307 There is a bit set aside in page->flags (PG_arch_1) as 308 "architecture private". The kernel guarantees that, 309 for pagecache pages, it will clear this bit when such 310 a page first enters the pagecache. 311 312 This allows these interfaces to be implemented much more 313 efficiently. It allows one to "defer" (perhaps indefinitely) 314 the actual flush if there are currently no user processes 315 mapping this page. See sparc64's flush_dcache_page and 316 update_mmu_cache implementations for an example of how to go 317 about doing this. 318 319 The idea is, first at flush_dcache_page() time, if 320 page->mapping->i_mmap is an empty tree, just mark the architecture 321 private page flag bit. Later, in update_mmu_cache(), a check is 322 made of this flag bit, and if set the flush is done and the flag 323 bit is cleared. 324 325 IMPORTANT NOTE: It is often important, if you defer the flush, 326 that the actual flush occurs on the same CPU 327 as did the cpu stores into the page to make it 328 dirty. Again, see sparc64 for examples of how 329 to deal with this. 330 331 void copy_to_user_page(struct vm_area_struct *vma, struct page *page, 332 unsigned long user_vaddr, 333 void *dst, void *src, int len) 334 void copy_from_user_page(struct vm_area_struct *vma, struct page *page, 335 unsigned long user_vaddr, 336 void *dst, void *src, int len) 337 When the kernel needs to copy arbitrary data in and out 338 of arbitrary user pages (f.e. for ptrace()) it will use 339 these two routines. 340 341 Any necessary cache flushing or other coherency operations 342 that need to occur should happen here. If the processor's 343 instruction cache does not snoop cpu stores, it is very 344 likely that you will need to flush the instruction cache 345 for copy_to_user_page(). 346 347 void flush_anon_page(struct vm_area_struct *vma, struct page *page, 348 unsigned long vmaddr) 349 When the kernel needs to access the contents of an anonymous 350 page, it calls this function (currently only 351 get_user_pages()). Note: flush_dcache_page() deliberately 352 doesn't work for an anonymous page. The default 353 implementation is a nop (and should remain so for all coherent 354 architectures). For incoherent architectures, it should flush 355 the cache of the page at vmaddr. 356 357 void flush_kernel_dcache_page(struct page *page) 358 When the kernel needs to modify a user page is has obtained 359 with kmap, it calls this function after all modifications are 360 complete (but before kunmapping it) to bring the underlying 361 page up to date. It is assumed here that the user has no 362 incoherent cached copies (i.e. the original page was obtained 363 from a mechanism like get_user_pages()). The default 364 implementation is a nop and should remain so on all coherent 365 architectures. On incoherent architectures, this should flush 366 the kernel cache for page (using page_address(page)). 367 368 369 void flush_icache_range(unsigned long start, unsigned long end) 370 When the kernel stores into addresses that it will execute 371 out of (eg when loading modules), this function is called. 372 373 If the icache does not snoop stores then this routine will need 374 to flush it. 375 376 void flush_icache_page(struct vm_area_struct *vma, struct page *page) 377 All the functionality of flush_icache_page can be implemented in 378 flush_dcache_page and update_mmu_cache. In the future, the hope 379 is to remove this interface completely. 380 381 The final category of APIs is for I/O to deliberately aliased address 382 ranges inside the kernel. Such aliases are set up by use of the 383 vmap/vmalloc API. Since kernel I/O goes via physical pages, the I/O 384 subsystem assumes that the user mapping and kernel offset mapping are 385 the only aliases. This isn't true for vmap aliases, so anything in 386 the kernel trying to do I/O to vmap areas must manually manage 387 coherency. It must do this by flushing the vmap range before doing 388 I/O and invalidating it after the I/O returns. 389 390 void flush_kernel_vmap_range(void *vaddr, int size) 391 flushes the kernel cache for a given virtual address range in 392 the vmap area. This is to make sure that any data the kernel 393 modified in the vmap range is made visible to the physical 394 page. The design is to make this area safe to perform I/O on. 395 Note that this API does *not* also flush the offset map alias 396 of the area. 397 398 void invalidate_kernel_vmap_range(void *vaddr, int size) invalidates 399 the cache for a given virtual address range in the vmap area 400 which prevents the processor from making the cache stale by 401 speculatively reading data while the I/O was occurring to the 402 physical pages. This is only necessary for data reads into the 403 vmap area.