[PATCH v3 3/5] arm64: mm: set the contiguous bit for kernel mappings where appropriate

[Catalin Marinas]

On Wed, Oct 12, 2016 at 12:23:43PM +0100, Ard Biesheuvel wrote:
> Now that we no longer allow live kernel PMDs to be split, it is safe to
> start using the contiguous bit for kernel mappings. So set the contiguous
> bit in the kernel page mappings for regions whose size and alignment are
> suitable for this.
> 
> This enables the following contiguous range sizes for the virtual mapping
> of the kernel image, and for the linear mapping:
> 
>           granule size |  cont PTE  |  cont PMD  |
>           -------------+------------+------------+
>                4 KB    |    64 KB   |   32 MB    |
>               16 KB    |     2 MB   |    1 GB*   |
>               64 KB    |     2 MB   |   16 GB*   |
> 
> * only when built for 3 or more levels of translation

I assume the limitation to have contiguous PMD only with 3 or move
levels is because of the way p*d folding was implemented in the kernel.
With nopmd, looping over pmds is done in __create_pgd_mapping() rather
than alloc_init_pmd().

A potential solution would be to replicate the contiguous pmd code to
the pud and pgd level, though we probably won't benefit from any
contiguous entries at higher level (when more than 2 levels).
Alternatively, with an #ifdef __PGTABLE_PMD_FOLDED, we could set the
PMD_CONT in prot in __create_pgd_mapping() directly (if the right
addr/phys alignment).

Anyway, it's probably not worth the effort given that 42-bit VA with 64K
pages is becoming a less likely configuration (36-bit VA with 16K pages
is even less likely, also depending on EXPERT).

-- 
Catalin