[PATCH 2/2] arm64: Extend early page table code to allow for larger kernels

[Mark Rutland]

Hi,

On Fri, Nov 17, 2017 at 11:41:43AM +0000, Steve Capper wrote:
> -#define SWAPPER_DIR_SIZE	(SWAPPER_PGTABLE_LEVELS * PAGE_SIZE)
> +#define EARLY_PGDS(vstart, vend) ((vend >> PGDIR_SHIFT) - (vstart >> PGDIR_SHIFT) + 1)
> +
> +#if SWAPPER_PGTABLE_LEVELS > 3
> +#define EARLY_PUDS(vstart, vend) ((vend >> PUD_SHIFT) - (vstart >> PUD_SHIFT) + 1)
> +#else
> +#define EARLY_PUDS(vstart, vend) (0)
> +#endif
> +
> +#if SWAPPER_PGTABLE_LEVELS > 2
> +#define EARLY_PMDS(vstart, vend) ((vend >> PMD_SHIFT) - (vstart >> PMD_SHIFT) + 1)
> +#else
> +#define EARLY_PMDS(vstart, vend) (0)
> +#endif
> +
> +#define EARLY_PAGES(vstart, vend) ( 1 			/* PGDIR page */				\
> +			+ EARLY_PGDS((vstart), (vend)) 	/* each PGDIR needs a next level page table */	\
> +			+ EARLY_PUDS((vstart), (vend))	/* each PUD needs a next level page table */	\
> +			+ EARLY_PMDS((vstart), (vend)))	/* each PMD needs a next level page table */
> +#define SWAPPER_DIR_SIZE (PAGE_SIZE * EARLY_PAGES(KIMAGE_VADDR + TEXT_OFFSET, _end + SZ_2M))
>  #define IDMAP_DIR_SIZE		(IDMAP_PGTABLE_LEVELS * PAGE_SIZE)

I'm currently struggling to convince myself as to whether 2M is
necessary/sufficient slack space for all configurations.

At least w.r.t. Ard's comment, we'd need a bit more slack to allow KASLR
to cross PGD/PUD/PMD boundaries.

For example with 3 levels of 64K pages, and a huge kernel that takes up
SZ_512M - SZ_2M - TEXT_OFFSET. That perfectly fits into 1 PGDIR, 1 PGD,
and 1 PMD. IIUC, the above would allocate us 3 pages for this case. If
the kernel were to be relocated such that it straddled two PGDs, we'd
need 2 PGDs and 2 PMDs, needing 5 pages total.

I'm not sure that we have a problem if we don't relax KASLR.

Thanks,
Mark.