contpte.c

  1// SPDX-License-Identifier: GPL-2.0-only
  2/*
  3 * Copyright (C) 2023 ARM Ltd.
  4 */
  5
  6#include <linux/mm.h>
  7#include <linux/efi.h>
  8#include <linux/export.h>
  9#include <asm/tlbflush.h>
 10
 11static inline bool mm_is_user(struct mm_struct *mm)
 12{
 13	/*
 14	 * Don't attempt to apply the contig bit to kernel mappings, because
 15	 * dynamically adding/removing the contig bit can cause page faults.
 16	 * These racing faults are ok for user space, since they get serialized
 17	 * on the PTL. But kernel mappings can't tolerate faults.
 18	 */
 19	if (unlikely(mm_is_efi(mm)))
 20		return false;
 21	return mm != &init_mm;
 22}
 23
 24static inline pte_t *contpte_align_down(pte_t *ptep)
 25{
 26	return PTR_ALIGN_DOWN(ptep, sizeof(*ptep) * CONT_PTES);
 27}
 28
 29static void contpte_try_unfold_partial(struct mm_struct *mm, unsigned long addr,
 30					pte_t *ptep, unsigned int nr)
 31{
 32	/*
 33	 * Unfold any partially covered contpte block at the beginning and end
 34	 * of the range.
 35	 */
 36
 37	if (ptep != contpte_align_down(ptep) || nr < CONT_PTES)
 38		contpte_try_unfold(mm, addr, ptep, __ptep_get(ptep));
 39
 40	if (ptep + nr != contpte_align_down(ptep + nr)) {
 41		unsigned long last_addr = addr + PAGE_SIZE * (nr - 1);
 42		pte_t *last_ptep = ptep + nr - 1;
 43
 44		contpte_try_unfold(mm, last_addr, last_ptep,
 45				   __ptep_get(last_ptep));
 46	}
 47}
 48
 49static void contpte_convert(struct mm_struct *mm, unsigned long addr,
 50			    pte_t *ptep, pte_t pte)
 51{
 52	struct vm_area_struct vma = TLB_FLUSH_VMA(mm, 0);
 53	unsigned long start_addr;
 54	pte_t *start_ptep;
 55	int i;
 56
 57	start_ptep = ptep = contpte_align_down(ptep);
 58	start_addr = addr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
 59	pte = pfn_pte(ALIGN_DOWN(pte_pfn(pte), CONT_PTES), pte_pgprot(pte));
 60
 61	for (i = 0; i < CONT_PTES; i++, ptep++, addr += PAGE_SIZE) {
 62		pte_t ptent = __ptep_get_and_clear(mm, addr, ptep);
 63
 64		if (pte_dirty(ptent))
 65			pte = pte_mkdirty(pte);
 66
 67		if (pte_young(ptent))
 68			pte = pte_mkyoung(pte);
 69	}
 70
 71	/*
 72	 * On eliding the __tlb_flush_range() under BBML2+noabort:
 73	 *
 74	 * NOTE: Instead of using N=16 as the contiguous block length, we use
 75	 *       N=4 for clarity.
 76	 *
 77	 * NOTE: 'n' and 'c' are used to denote the "contiguous bit" being
 78	 *       unset and set, respectively.
 79	 *
 80	 * We worry about two cases where contiguous bit is used:
 81	 *  - When folding N smaller non-contiguous ptes as 1 contiguous block.
 82	 *  - When unfolding a contiguous block into N smaller non-contiguous ptes.
 83	 *
 84	 * Currently, the BBML0 folding case looks as follows:
 85	 *
 86	 *  0) Initial page-table layout:
 87	 *
 88	 *   +----+----+----+----+
 89	 *   |RO,n|RO,n|RO,n|RW,n| <--- last page being set as RO
 90	 *   +----+----+----+----+
 91	 *
 92	 *  1) Aggregate AF + dirty flags using __ptep_get_and_clear():
 93	 *
 94	 *   +----+----+----+----+
 95	 *   |  0 |  0 |  0 |  0 |
 96	 *   +----+----+----+----+
 97	 *
 98	 *  2) __flush_tlb_range():
 99	 *
100	 *   |____ tlbi + dsb ____|
101	 *
102	 *  3) __set_ptes() to repaint contiguous block:
103	 *
104	 *   +----+----+----+----+
105	 *   |RO,c|RO,c|RO,c|RO,c|
106	 *   +----+----+----+----+
107	 *
108	 *  4) The kernel will eventually __flush_tlb() for changed page:
109	 *
110	 *                  |____| <--- tlbi + dsb
111	 *
112	 * As expected, the intermediate tlbi+dsb ensures that other PEs
113	 * only ever see an invalid (0) entry, or the new contiguous TLB entry.
114	 * The final tlbi+dsb will always throw away the newly installed
115	 * contiguous TLB entry, which is a micro-optimisation opportunity,
116	 * but does not affect correctness.
117	 *
118	 * In the BBML2 case, the change is avoiding the intermediate tlbi+dsb.
119	 * This means a few things, but notably other PEs will still "see" any
120	 * stale cached TLB entries. This could lead to a "contiguous bit
121	 * misprogramming" issue until the final tlbi+dsb of the changed page,
122	 * which would clear out both the stale (RW,n) entry and the new (RO,c)
123	 * contiguous entry installed in its place.
124	 *
125	 * What this is saying, is the following:
126	 *
127	 *  +----+----+----+----+
128	 *  |RO,n|RO,n|RO,n|RW,n| <--- old page tables, all non-contiguous
129	 *  +----+----+----+----+
130	 *
131	 *  +----+----+----+----+
132	 *  |RO,c|RO,c|RO,c|RO,c| <--- new page tables, all contiguous
133	 *  +----+----+----+----+
134	 *   /\
135	 *   ||
136	 *
137	 *  If both the old single (RW,n) and new contiguous (RO,c) TLB entries
138	 *  are present, and a write is made to this address, do we fault or
139	 *  is the write permitted (via amalgamation)?
140	 *
141	 * The relevant Arm ARM DDI 0487L.a requirements are RNGLXZ and RJQQTC,
142	 * and together state that when BBML1 or BBML2 are implemented, either
143	 * a TLB conflict abort is raised (which we expressly forbid), or will
144	 * "produce an OA, access permissions, and memory attributes that are
145	 * consistent with any of the programmed translation table values".
146	 *
147	 * That is to say, will either raise a TLB conflict, or produce one of
148	 * the cached TLB entries, but never amalgamate.
149	 *
150	 * Thus, as the page tables are only considered "consistent" after
151	 * the final tlbi+dsb (which evicts both the single stale (RW,n) TLB
152	 * entry as well as the new contiguous (RO,c) TLB entry), omitting the
153	 * initial tlbi+dsb is correct.
154	 *
155	 * It is also important to note that at the end of the BBML2 folding
156	 * case, we are still left with potentially all N TLB entries still
157	 * cached (the N-1 non-contiguous ptes, and the single contiguous
158	 * block). However, over time, natural TLB pressure will cause the
159	 * non-contiguous pte TLB entries to be flushed, leaving only the
160	 * contiguous block TLB entry. This means that omitting the tlbi+dsb is
161	 * not only correct, but also keeps our eventual performance benefits.
162	 *
163	 * For the unfolding case, BBML0 looks as follows:
164	 *
165	 *  0) Initial page-table layout:
166	 *
167	 *   +----+----+----+----+
168	 *   |RW,c|RW,c|RW,c|RW,c| <--- last page being set as RO
169	 *   +----+----+----+----+
170	 *
171	 *  1) Aggregate AF + dirty flags using __ptep_get_and_clear():
172	 *
173	 *   +----+----+----+----+
174	 *   |  0 |  0 |  0 |  0 |
175	 *   +----+----+----+----+
176	 *
177	 *  2) __flush_tlb_range():
178	 *
179	 *   |____ tlbi + dsb ____|
180	 *
181	 *  3) __set_ptes() to repaint as non-contiguous:
182	 *
183	 *   +----+----+----+----+
184	 *   |RW,n|RW,n|RW,n|RW,n|
185	 *   +----+----+----+----+
186	 *
187	 *  4) Update changed page permissions:
188	 *
189	 *   +----+----+----+----+
190	 *   |RW,n|RW,n|RW,n|RO,n| <--- last page permissions set
191	 *   +----+----+----+----+
192	 *
193	 *  5) The kernel will eventually __flush_tlb() for changed page:
194	 *
195	 *                  |____| <--- tlbi + dsb
196	 *
197	 * For BBML2, we again remove the intermediate tlbi+dsb. Here, there
198	 * are no issues, as the final tlbi+dsb covering the changed page is
199	 * guaranteed to remove the original large contiguous (RW,c) TLB entry,
200	 * as well as the intermediate (RW,n) TLB entry; the next access will
201	 * install the new (RO,n) TLB entry and the page tables are only
202	 * considered "consistent" after the final tlbi+dsb, so software must
203	 * be prepared for this inconsistency prior to finishing the mm dance
204	 * regardless.
205	 */
206
207	if (!system_supports_bbml2_noabort())
208		__flush_tlb_range(&vma, start_addr, addr, PAGE_SIZE, true, 3);
209
210	__set_ptes(mm, start_addr, start_ptep, pte, CONT_PTES);
211}
212
213void __contpte_try_fold(struct mm_struct *mm, unsigned long addr,
214			pte_t *ptep, pte_t pte)
215{
216	/*
217	 * We have already checked that the virtual and pysical addresses are
218	 * correctly aligned for a contpte mapping in contpte_try_fold() so the
219	 * remaining checks are to ensure that the contpte range is fully
220	 * covered by a single folio, and ensure that all the ptes are valid
221	 * with contiguous PFNs and matching prots. We ignore the state of the
222	 * access and dirty bits for the purpose of deciding if its a contiguous
223	 * range; the folding process will generate a single contpte entry which
224	 * has a single access and dirty bit. Those 2 bits are the logical OR of
225	 * their respective bits in the constituent pte entries. In order to
226	 * ensure the contpte range is covered by a single folio, we must
227	 * recover the folio from the pfn, but special mappings don't have a
228	 * folio backing them. Fortunately contpte_try_fold() already checked
229	 * that the pte is not special - we never try to fold special mappings.
230	 * Note we can't use vm_normal_page() for this since we don't have the
231	 * vma.
232	 */
233
234	unsigned long folio_start, folio_end;
235	unsigned long cont_start, cont_end;
236	pte_t expected_pte, subpte;
237	struct folio *folio;
238	struct page *page;
239	unsigned long pfn;
240	pte_t *orig_ptep;
241	pgprot_t prot;
242
243	int i;
244
245	if (!mm_is_user(mm))
246		return;
247
248	page = pte_page(pte);
249	folio = page_folio(page);
250	folio_start = addr - (page - &folio->page) * PAGE_SIZE;
251	folio_end = folio_start + folio_nr_pages(folio) * PAGE_SIZE;
252	cont_start = ALIGN_DOWN(addr, CONT_PTE_SIZE);
253	cont_end = cont_start + CONT_PTE_SIZE;
254
255	if (folio_start > cont_start || folio_end < cont_end)
256		return;
257
258	pfn = ALIGN_DOWN(pte_pfn(pte), CONT_PTES);
259	prot = pte_pgprot(pte_mkold(pte_mkclean(pte)));
260	expected_pte = pfn_pte(pfn, prot);
261	orig_ptep = ptep;
262	ptep = contpte_align_down(ptep);
263
264	for (i = 0; i < CONT_PTES; i++) {
265		subpte = pte_mkold(pte_mkclean(__ptep_get(ptep)));
266		if (!pte_same(subpte, expected_pte))
267			return;
268		expected_pte = pte_advance_pfn(expected_pte, 1);
269		ptep++;
270	}
271
272	pte = pte_mkcont(pte);
273	contpte_convert(mm, addr, orig_ptep, pte);
274}
275EXPORT_SYMBOL_GPL(__contpte_try_fold);
276
277void __contpte_try_unfold(struct mm_struct *mm, unsigned long addr,
278			pte_t *ptep, pte_t pte)
279{
280	/*
281	 * We have already checked that the ptes are contiguous in
282	 * contpte_try_unfold(), so just check that the mm is user space.
283	 */
284	if (!mm_is_user(mm))
285		return;
286
287	pte = pte_mknoncont(pte);
288	contpte_convert(mm, addr, ptep, pte);
289}
290EXPORT_SYMBOL_GPL(__contpte_try_unfold);
291
292pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte)
293{
294	/*
295	 * Gather access/dirty bits, which may be populated in any of the ptes
296	 * of the contig range. We are guaranteed to be holding the PTL, so any
297	 * contiguous range cannot be unfolded or otherwise modified under our
298	 * feet.
299	 */
300
301	pte_t pte;
302	int i;
303
304	ptep = contpte_align_down(ptep);
305
306	for (i = 0; i < CONT_PTES; i++, ptep++) {
307		pte = __ptep_get(ptep);
308
309		if (pte_dirty(pte)) {
310			orig_pte = pte_mkdirty(orig_pte);
311			for (; i < CONT_PTES; i++, ptep++) {
312				pte = __ptep_get(ptep);
313				if (pte_young(pte)) {
314					orig_pte = pte_mkyoung(orig_pte);
315					break;
316				}
317			}
318			break;
319		}
320
321		if (pte_young(pte)) {
322			orig_pte = pte_mkyoung(orig_pte);
323			i++;
324			ptep++;
325			for (; i < CONT_PTES; i++, ptep++) {
326				pte = __ptep_get(ptep);
327				if (pte_dirty(pte)) {
328					orig_pte = pte_mkdirty(orig_pte);
329					break;
330				}
331			}
332			break;
333		}
334	}
335
336	return orig_pte;
337}
338EXPORT_SYMBOL_GPL(contpte_ptep_get);
339
340static inline bool contpte_is_consistent(pte_t pte, unsigned long pfn,
341					pgprot_t orig_prot)
342{
343	pgprot_t prot = pte_pgprot(pte_mkold(pte_mkclean(pte)));
344
345	return pte_valid_cont(pte) && pte_pfn(pte) == pfn &&
346			pgprot_val(prot) == pgprot_val(orig_prot);
347}
348
349pte_t contpte_ptep_get_lockless(pte_t *orig_ptep)
350{
351	/*
352	 * The ptep_get_lockless() API requires us to read and return *orig_ptep
353	 * so that it is self-consistent, without the PTL held, so we may be
354	 * racing with other threads modifying the pte. Usually a READ_ONCE()
355	 * would suffice, but for the contpte case, we also need to gather the
356	 * access and dirty bits from across all ptes in the contiguous block,
357	 * and we can't read all of those neighbouring ptes atomically, so any
358	 * contiguous range may be unfolded/modified/refolded under our feet.
359	 * Therefore we ensure we read a _consistent_ contpte range by checking
360	 * that all ptes in the range are valid and have CONT_PTE set, that all
361	 * pfns are contiguous and that all pgprots are the same (ignoring
362	 * access/dirty). If we find a pte that is not consistent, then we must
363	 * be racing with an update so start again. If the target pte does not
364	 * have CONT_PTE set then that is considered consistent on its own
365	 * because it is not part of a contpte range.
366	 */
367
368	pgprot_t orig_prot;
369	unsigned long pfn;
370	pte_t orig_pte;
371	pte_t *ptep;
372	pte_t pte;
373	int i;
374
375retry:
376	orig_pte = __ptep_get(orig_ptep);
377
378	if (!pte_valid_cont(orig_pte))
379		return orig_pte;
380
381	orig_prot = pte_pgprot(pte_mkold(pte_mkclean(orig_pte)));
382	ptep = contpte_align_down(orig_ptep);
383	pfn = pte_pfn(orig_pte) - (orig_ptep - ptep);
384
385	for (i = 0; i < CONT_PTES; i++, ptep++, pfn++) {
386		pte = __ptep_get(ptep);
387
388		if (!contpte_is_consistent(pte, pfn, orig_prot))
389			goto retry;
390
391		if (pte_dirty(pte)) {
392			orig_pte = pte_mkdirty(orig_pte);
393			for (; i < CONT_PTES; i++, ptep++, pfn++) {
394				pte = __ptep_get(ptep);
395
396				if (!contpte_is_consistent(pte, pfn, orig_prot))
397					goto retry;
398
399				if (pte_young(pte)) {
400					orig_pte = pte_mkyoung(orig_pte);
401					break;
402				}
403			}
404			break;
405		}
406
407		if (pte_young(pte)) {
408			orig_pte = pte_mkyoung(orig_pte);
409			i++;
410			ptep++;
411			pfn++;
412			for (; i < CONT_PTES; i++, ptep++, pfn++) {
413				pte = __ptep_get(ptep);
414
415				if (!contpte_is_consistent(pte, pfn, orig_prot))
416					goto retry;
417
418				if (pte_dirty(pte)) {
419					orig_pte = pte_mkdirty(orig_pte);
420					break;
421				}
422			}
423			break;
424		}
425	}
426
427	return orig_pte;
428}
429EXPORT_SYMBOL_GPL(contpte_ptep_get_lockless);
430
431void contpte_set_ptes(struct mm_struct *mm, unsigned long addr,
432					pte_t *ptep, pte_t pte, unsigned int nr)
433{
434	unsigned long next;
435	unsigned long end;
436	unsigned long pfn;
437	pgprot_t prot;
438
439	/*
440	 * The set_ptes() spec guarantees that when nr > 1, the initial state of
441	 * all ptes is not-present. Therefore we never need to unfold or
442	 * otherwise invalidate a range before we set the new ptes.
443	 * contpte_set_ptes() should never be called for nr < 2.
444	 */
445	VM_WARN_ON(nr == 1);
446
447	if (!mm_is_user(mm))
448		return __set_ptes(mm, addr, ptep, pte, nr);
449
450	end = addr + (nr << PAGE_SHIFT);
451	pfn = pte_pfn(pte);
452	prot = pte_pgprot(pte);
453
454	do {
455		next = pte_cont_addr_end(addr, end);
456		nr = (next - addr) >> PAGE_SHIFT;
457		pte = pfn_pte(pfn, prot);
458
459		if (((addr | next | (pfn << PAGE_SHIFT)) & ~CONT_PTE_MASK) == 0)
460			pte = pte_mkcont(pte);
461		else
462			pte = pte_mknoncont(pte);
463
464		__set_ptes(mm, addr, ptep, pte, nr);
465
466		addr = next;
467		ptep += nr;
468		pfn += nr;
469
470	} while (addr != end);
471}
472EXPORT_SYMBOL_GPL(contpte_set_ptes);
473
474void contpte_clear_full_ptes(struct mm_struct *mm, unsigned long addr,
475				pte_t *ptep, unsigned int nr, int full)
476{
477	contpte_try_unfold_partial(mm, addr, ptep, nr);
478	__clear_full_ptes(mm, addr, ptep, nr, full);
479}
480EXPORT_SYMBOL_GPL(contpte_clear_full_ptes);
481
482pte_t contpte_get_and_clear_full_ptes(struct mm_struct *mm,
483				unsigned long addr, pte_t *ptep,
484				unsigned int nr, int full)
485{
486	contpte_try_unfold_partial(mm, addr, ptep, nr);
487	return __get_and_clear_full_ptes(mm, addr, ptep, nr, full);
488}
489EXPORT_SYMBOL_GPL(contpte_get_and_clear_full_ptes);
490
491int contpte_ptep_test_and_clear_young(struct vm_area_struct *vma,
492					unsigned long addr, pte_t *ptep)
493{
494	/*
495	 * ptep_clear_flush_young() technically requires us to clear the access
496	 * flag for a _single_ pte. However, the core-mm code actually tracks
497	 * access/dirty per folio, not per page. And since we only create a
498	 * contig range when the range is covered by a single folio, we can get
499	 * away with clearing young for the whole contig range here, so we avoid
500	 * having to unfold.
501	 */
502
503	int young = 0;
504	int i;
505
506	ptep = contpte_align_down(ptep);
507	addr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
508
509	for (i = 0; i < CONT_PTES; i++, ptep++, addr += PAGE_SIZE)
510		young |= __ptep_test_and_clear_young(vma, addr, ptep);
511
512	return young;
513}
514EXPORT_SYMBOL_GPL(contpte_ptep_test_and_clear_young);
515
516int contpte_ptep_clear_flush_young(struct vm_area_struct *vma,
517					unsigned long addr, pte_t *ptep)
518{
519	int young;
520
521	young = contpte_ptep_test_and_clear_young(vma, addr, ptep);
522
523	if (young) {
524		/*
525		 * See comment in __ptep_clear_flush_young(); same rationale for
526		 * eliding the trailing DSB applies here.
527		 */
528		addr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
529		__flush_tlb_range_nosync(vma->vm_mm, addr, addr + CONT_PTE_SIZE,
530					 PAGE_SIZE, true, 3);
531	}
532
533	return young;
534}
535EXPORT_SYMBOL_GPL(contpte_ptep_clear_flush_young);
536
537void contpte_wrprotect_ptes(struct mm_struct *mm, unsigned long addr,
538					pte_t *ptep, unsigned int nr)
539{
540	/*
541	 * If wrprotecting an entire contig range, we can avoid unfolding. Just
542	 * set wrprotect and wait for the later mmu_gather flush to invalidate
543	 * the tlb. Until the flush, the page may or may not be wrprotected.
544	 * After the flush, it is guaranteed wrprotected. If it's a partial
545	 * range though, we must unfold, because we can't have a case where
546	 * CONT_PTE is set but wrprotect applies to a subset of the PTEs; this
547	 * would cause it to continue to be unpredictable after the flush.
548	 */
549
550	contpte_try_unfold_partial(mm, addr, ptep, nr);
551	__wrprotect_ptes(mm, addr, ptep, nr);
552}
553EXPORT_SYMBOL_GPL(contpte_wrprotect_ptes);
554
555void contpte_clear_young_dirty_ptes(struct vm_area_struct *vma,
556				    unsigned long addr, pte_t *ptep,
557				    unsigned int nr, cydp_t flags)
558{
559	/*
560	 * We can safely clear access/dirty without needing to unfold from
561	 * the architectures perspective, even when contpte is set. If the
562	 * range starts or ends midway through a contpte block, we can just
563	 * expand to include the full contpte block. While this is not
564	 * exactly what the core-mm asked for, it tracks access/dirty per
565	 * folio, not per page. And since we only create a contpte block
566	 * when it is covered by a single folio, we can get away with
567	 * clearing access/dirty for the whole block.
568	 */
569	unsigned long start = addr;
570	unsigned long end = start + nr * PAGE_SIZE;
571
572	if (pte_cont(__ptep_get(ptep + nr - 1)))
573		end = ALIGN(end, CONT_PTE_SIZE);
574
575	if (pte_cont(__ptep_get(ptep))) {
576		start = ALIGN_DOWN(start, CONT_PTE_SIZE);
577		ptep = contpte_align_down(ptep);
578	}
579
580	__clear_young_dirty_ptes(vma, start, ptep, (end - start) / PAGE_SIZE, flags);
581}
582EXPORT_SYMBOL_GPL(contpte_clear_young_dirty_ptes);
583
584static bool contpte_all_subptes_match_access_flags(pte_t *ptep, pte_t entry)
585{
586	pte_t *cont_ptep = contpte_align_down(ptep);
587	/*
588	 * PFNs differ per sub-PTE. Match only bits consumed by
589	 * __ptep_set_access_flags(): AF, DIRTY and write permission.
590	 */
591	const pteval_t cmp_mask = PTE_RDONLY | PTE_AF | PTE_WRITE | PTE_DIRTY;
592	pteval_t entry_cmp = pte_val(entry) & cmp_mask;
593	int i;
594
595	for (i = 0; i < CONT_PTES; i++) {
596		pteval_t pte_cmp = pte_val(__ptep_get(cont_ptep + i)) & cmp_mask;
597
598		if (pte_cmp != entry_cmp)
599			return false;
600	}
601
602	return true;
603}
604
605int contpte_ptep_set_access_flags(struct vm_area_struct *vma,
606					unsigned long addr, pte_t *ptep,
607					pte_t entry, int dirty)
608{
609	unsigned long start_addr;
610	pte_t orig_pte;
611	int i;
612
613	/*
614	 * Check whether all sub-PTEs in the CONT block already match the
615	 * requested access flags/write permission, using raw per-PTE values
616	 * rather than the gathered ptep_get() view.
617	 *
618	 * __ptep_set_access_flags() can update AF, dirty and write
619	 * permission, but only to make the mapping more permissive.
620	 *
621	 * ptep_get() gathers AF/dirty state across the whole CONT block,
622	 * which is correct for a CPU with FEAT_HAFDBS. But page-table
623	 * walkers that evaluate each descriptor individually (e.g. a CPU
624	 * without DBM support, or an SMMU without HTTU, or with HA/HD
625	 * disabled in CD.TCR) can keep faulting on the target sub-PTE if
626	 * only a sibling has been updated. Gathering can therefore cause
627	 * false no-ops when only a sibling has been updated:
628	 *  - write faults: target still has PTE_RDONLY (needs PTE_RDONLY cleared)
629	 *  - read faults:  target still lacks PTE_AF
630	 *
631	 * Per Arm ARM (DDI 0487) D8.7.1, any sub-PTE in a CONT range may
632	 * become the effective cached translation, so all entries must have
633	 * consistent attributes. Check the full CONT block before returning
634	 * no-op, and when any sub-PTE mismatches, proceed to update the whole
635	 * range.
636	 */
637	if (contpte_all_subptes_match_access_flags(ptep, entry))
638		return 0;
639
640	/*
641	 * Use raw target pte (not gathered) for write-bit unfold decision.
642	 */
643	orig_pte = pte_mknoncont(__ptep_get(ptep));
644
645	/*
646	 * We can fix up access/dirty bits without having to unfold the contig
647	 * range. But if the write bit is changing, we must unfold.
648	 */
649	if (pte_write(orig_pte) == pte_write(entry)) {
650		/*
651		 * For HW access management, we technically only need to update
652		 * the flag on a single pte in the range. But for SW access
653		 * management, we need to update all the ptes to prevent extra
654		 * faults. Avoid per-page tlb flush in __ptep_set_access_flags()
655		 * and instead flush the whole range at the end.
656		 */
657		ptep = contpte_align_down(ptep);
658		start_addr = addr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
659
660		/*
661		 * We are not advancing entry because __ptep_set_access_flags()
662		 * only consumes access flags from entry. And since we have checked
663		 * for the whole contpte block and returned early, pte_same()
664		 * within __ptep_set_access_flags() is likely false.
665		 */
666		for (i = 0; i < CONT_PTES; i++, ptep++, addr += PAGE_SIZE)
667			__ptep_set_access_flags(vma, addr, ptep, entry, 0);
668
669		if (dirty)
670			local_flush_tlb_contpte(vma, start_addr);
671	} else {
672		__contpte_try_unfold(vma->vm_mm, addr, ptep, orig_pte);
673		__ptep_set_access_flags(vma, addr, ptep, entry, dirty);
674	}
675
676	return 1;
677}
678EXPORT_SYMBOL_GPL(contpte_ptep_set_access_flags);