Re: [PATCH v3 32/60] arm64: head: allocate more pages for the kernel mapping

Ryan Roberts
2023-04-17T15:48:14Z

On 07/03/2023 14:04, Ard Biesheuvel wrote:
> In preparation for switching to an early kernel mapping routine that
> maps each segment according to its precise boundaries, and with the
> correct attributes, let's allocate some extra pages for page tables for
> the 4k page size configuration. This is necessary because the start and
> end of each segment may not be aligned to the block size, and so we'll
> need an extra page table at each segment boundary.
>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
> arch/arm64/include/asm/kernel-pgtable.h | 11 ++++++++++-
> 1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/include/asm/kernel-pgtable.h b/arch/arm64/include/asm/kernel-pgtable.h
> index 4d13c73171e1e360..50b5c145358a5d8e 100644
> --- a/arch/arm64/include/asm/kernel-pgtable.h
> +++ b/arch/arm64/include/asm/kernel-pgtable.h
> @@ -80,7 +80,7 @@
> + EARLY_PGDS((vstart), (vend), add) /* each PGDIR needs a next level page table */ \
> + EARLY_PUDS((vstart), (vend), add) /* each PUD needs a next level page table */ \
> + EARLY_PMDS((vstart), (vend), add)) /* each PMD needs a next level page table */
> -#define INIT_DIR_SIZE (PAGE_SIZE * EARLY_PAGES(KIMAGE_VADDR, _end, EARLY_KASLR))
> +#define INIT_DIR_SIZE (PAGE_SIZE * (EARLY_PAGES(KIMAGE_VADDR, _end, EARLY_KASLR) + EARLY_SEGMENT_EXTRA_PAGES))
>
> /* the initial ID map may need two extra pages if it needs to be extended */
> #if VA_BITS < 48
> @@ -101,6 +101,15 @@
> #define SWAPPER_TABLE_SHIFT PMD_SHIFT
> #endif
>
> +/* The number of segments in the kernel image (text, rodata, inittext, initdata, data+bss) */
> +#define KERNEL_SEGMENT_COUNT 5
> +
> +#if SWAPPER_BLOCK_SIZE > SEGMENT_ALIGN
> +#define EARLY_SEGMENT_EXTRA_PAGES (KERNEL_SEGMENT_COUNT + 1)

I'm guessing the block size for 4K pages is PMD, so you need these extra pages
to define PTEs for the case where the section start/end addresses are not on
exact 2MB boundaries? But in that case, isn't it possible that you would need 2
extra PTE tables per segment - one for the start and one for the end?

> +#else
> +#define EARLY_SEGMENT_EXTRA_PAGES 0
> +#endif
> +
> /*
> * Initial memory map attributes.
> */