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SubjectRe: [PATCH v1 2/4] ioremap: Invalidate TLB after huge mappings
On Wed, Mar 14, 2018 at 02:18:23PM +0530, Chintan Pandya wrote:
> If huge mappings are enabled, they can override
> valid intermediate previous mappings. Some MMU
> can speculatively pre-fetch these intermediate
> entries even after unmap. That's because unmap
> will clear only last level entries in page table
> keeping intermediate (pud/pmd) entries still valid.
>
> This can potentially lead to stale TLB entries
> which needs invalidation after map.
>
> Some more info: https://lkml.org/lkml/2017/12/23/3
>
> There is one noted case for ARM64 where such stale
> TLB entries causes 3rd level translation fault even
> after correct (huge) mapping is available.

> Hence, invalidate once we override pmd/pud with huge
> mappings.

> static int __read_mostly ioremap_p4d_capable;
> @@ -92,8 +93,10 @@ static inline int ioremap_pmd_range(pud_t *pud, unsigned long addr,
> if (ioremap_pmd_enabled() &&
> ((next - addr) == PMD_SIZE) &&
> IS_ALIGNED(phys_addr + addr, PMD_SIZE)) {
> - if (pmd_set_huge(pmd, phys_addr + addr, prot))
> + if (pmd_set_huge(pmd, phys_addr + addr, prot)) {
> + flush_tlb_pgtable(&init_mm, addr);
> continue;
> + }
> }
>
> if (ioremap_pte_range(pmd, addr, next, phys_addr + addr, prot))
> @@ -118,8 +121,10 @@ static inline int ioremap_pud_range(p4d_t *p4d, unsigned long addr,
> if (ioremap_pud_enabled() &&
> ((next - addr) == PUD_SIZE) &&
> IS_ALIGNED(phys_addr + addr, PUD_SIZE)) {
> - if (pud_set_huge(pud, phys_addr + addr, prot))
> + if (pud_set_huge(pud, phys_addr + addr, prot)) {
> + flush_tlb_pgtable(&init_mm, addr);
> continue;
> + }
> }

As has been noted in previous threads, the ARM architecture requires a
Break-Before-Make sequence when changing an entry from a table to a
block, as is the case here.

The means the necessary sequence is:

1. Make the entry invalid
2. Invalidate relevant TLB entries
3. Write the new entry

Whereas above, the sequence is

1. Write the new entry
2. invalidate relevant TLB entries

Which is insufficient, and will lead to a number of problems.

Therefore, NAK to this patch.

Please read up on the Break-Before-Make requirements in the ARM ARM.

Thanks,
Mark.

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