diff options
| -rw-r--r-- | arch/arm64/mm/contpte.c | 139 |
1 files changed, 138 insertions, 1 deletions
diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c index bcac4f55f9c1..203357061d0a 100644 --- a/arch/arm64/mm/contpte.c +++ b/arch/arm64/mm/contpte.c @@ -68,7 +68,144 @@ static void contpte_convert(struct mm_struct *mm, unsigned long addr, pte = pte_mkyoung(pte); } - __flush_tlb_range(&vma, start_addr, addr, PAGE_SIZE, true, 3); + /* + * On eliding the __tlb_flush_range() under BBML2+noabort: + * + * NOTE: Instead of using N=16 as the contiguous block length, we use + * N=4 for clarity. + * + * NOTE: 'n' and 'c' are used to denote the "contiguous bit" being + * unset and set, respectively. + * + * We worry about two cases where contiguous bit is used: + * - When folding N smaller non-contiguous ptes as 1 contiguous block. + * - When unfolding a contiguous block into N smaller non-contiguous ptes. + * + * Currently, the BBML0 folding case looks as follows: + * + * 0) Initial page-table layout: + * + * +----+----+----+----+ + * |RO,n|RO,n|RO,n|RW,n| <--- last page being set as RO + * +----+----+----+----+ + * + * 1) Aggregate AF + dirty flags using __ptep_get_and_clear(): + * + * +----+----+----+----+ + * | 0 | 0 | 0 | 0 | + * +----+----+----+----+ + * + * 2) __flush_tlb_range(): + * + * |____ tlbi + dsb ____| + * + * 3) __set_ptes() to repaint contiguous block: + * + * +----+----+----+----+ + * |RO,c|RO,c|RO,c|RO,c| + * +----+----+----+----+ + * + * 4) The kernel will eventually __flush_tlb() for changed page: + * + * |____| <--- tlbi + dsb + * + * As expected, the intermediate tlbi+dsb ensures that other PEs + * only ever see an invalid (0) entry, or the new contiguous TLB entry. + * The final tlbi+dsb will always throw away the newly installed + * contiguous TLB entry, which is a micro-optimisation opportunity, + * but does not affect correctness. + * + * In the BBML2 case, the change is avoiding the intermediate tlbi+dsb. + * This means a few things, but notably other PEs will still "see" any + * stale cached TLB entries. This could lead to a "contiguous bit + * misprogramming" issue until the final tlbi+dsb of the changed page, + * which would clear out both the stale (RW,n) entry and the new (RO,c) + * contiguous entry installed in its place. + * + * What this is saying, is the following: + * + * +----+----+----+----+ + * |RO,n|RO,n|RO,n|RW,n| <--- old page tables, all non-contiguous + * +----+----+----+----+ + * + * +----+----+----+----+ + * |RO,c|RO,c|RO,c|RO,c| <--- new page tables, all contiguous + * +----+----+----+----+ + * /\ + * || + * + * If both the old single (RW,n) and new contiguous (RO,c) TLB entries + * are present, and a write is made to this address, do we fault or + * is the write permitted (via amalgamation)? + * + * The relevant Arm ARM DDI 0487L.a requirements are RNGLXZ and RJQQTC, + * and together state that when BBML1 or BBML2 are implemented, either + * a TLB conflict abort is raised (which we expressly forbid), or will + * "produce an OA, access permissions, and memory attributes that are + * consistent with any of the programmed translation table values". + * + * That is to say, will either raise a TLB conflict, or produce one of + * the cached TLB entries, but never amalgamate. + * + * Thus, as the page tables are only considered "consistent" after + * the final tlbi+dsb (which evicts both the single stale (RW,n) TLB + * entry as well as the new contiguous (RO,c) TLB entry), omitting the + * initial tlbi+dsb is correct. + * + * It is also important to note that at the end of the BBML2 folding + * case, we are still left with potentially all N TLB entries still + * cached (the N-1 non-contiguous ptes, and the single contiguous + * block). However, over time, natural TLB pressure will cause the + * non-contiguous pte TLB entries to be flushed, leaving only the + * contiguous block TLB entry. This means that omitting the tlbi+dsb is + * not only correct, but also keeps our eventual performance benefits. + * + * For the unfolding case, BBML0 looks as follows: + * + * 0) Initial page-table layout: + * + * +----+----+----+----+ + * |RW,c|RW,c|RW,c|RW,c| <--- last page being set as RO + * +----+----+----+----+ + * + * 1) Aggregate AF + dirty flags using __ptep_get_and_clear(): + * + * +----+----+----+----+ + * | 0 | 0 | 0 | 0 | + * +----+----+----+----+ + * + * 2) __flush_tlb_range(): + * + * |____ tlbi + dsb ____| + * + * 3) __set_ptes() to repaint as non-contiguous: + * + * +----+----+----+----+ + * |RW,n|RW,n|RW,n|RW,n| + * +----+----+----+----+ + * + * 4) Update changed page permissions: + * + * +----+----+----+----+ + * |RW,n|RW,n|RW,n|RO,n| <--- last page permissions set + * +----+----+----+----+ + * + * 5) The kernel will eventually __flush_tlb() for changed page: + * + * |____| <--- tlbi + dsb + * + * For BBML2, we again remove the intermediate tlbi+dsb. Here, there + * are no issues, as the final tlbi+dsb covering the changed page is + * guaranteed to remove the original large contiguous (RW,c) TLB entry, + * as well as the intermediate (RW,n) TLB entry; the next access will + * install the new (RO,n) TLB entry and the page tables are only + * considered "consistent" after the final tlbi+dsb, so software must + * be prepared for this inconsistency prior to finishing the mm dance + * regardless. + */ + + if (!system_supports_bbml2_noabort()) + __flush_tlb_range(&vma, start_addr, addr, PAGE_SIZE, true, 3); __set_ptes(mm, start_addr, start_ptep, pte, CONT_PTES); } |