[PATCH] arm64: relax Image placement requirement

[Ard Biesheuvel]

On 20 March 2015 at 18:02, Ard Biesheuvel <ard.biesheuvel at linaro.org> wrote:
> Hi all,
>
> This is another possible approach to fixing the current, flawed
> EFI Image placement logic: instead of fixing that logic and the
> documentation, why not relax the requirement that the kernel
> Image not be placed across a 512 MB boundary?
>

Hmm, it seems this still needs a pinch of refinement, but we could at
least discuss the general idea.

-- 
Ard.

> ---------------->8-----------------
>
> This patch changes the early page table code so that two adjacent
> entries are used to map two pages worth of block entries at the
> lowest level, both for idmap_pg_dir and swapper_pg_dir.
>
> The purpose is to allow the kernel Image to cross a 512 MB or 1 GB
> alignment boundary (depending on page size), which is something that
> is not specifically banned by the current wording of the boot protocol.
> (The boot protocol stipulates that the kernel must be placed within
> 512 MB if the beginning of RAM. However, the beginning of RAM is not
> necessarily aligned to 512 MB)
>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
> ---
>  arch/arm64/include/asm/page.h |  4 ++--
>  arch/arm64/kernel/head.S      | 17 +++++++++++++----
>  2 files changed, 15 insertions(+), 6 deletions(-)
>
> diff --git a/arch/arm64/include/asm/page.h b/arch/arm64/include/asm/page.h
> index 3d02b1869eb8..d2189c359364 100644
> --- a/arch/arm64/include/asm/page.h
> +++ b/arch/arm64/include/asm/page.h
> @@ -43,8 +43,8 @@
>  #define SWAPPER_PGTABLE_LEVELS (CONFIG_ARM64_PGTABLE_LEVELS - 1)
>  #endif
>
> -#define SWAPPER_DIR_SIZE       (SWAPPER_PGTABLE_LEVELS * PAGE_SIZE)
> -#define IDMAP_DIR_SIZE         (3 * PAGE_SIZE)
> +#define SWAPPER_DIR_SIZE       ((SWAPPER_PGTABLE_LEVELS + 1) * PAGE_SIZE)
> +#define IDMAP_DIR_SIZE         (4 * PAGE_SIZE)
>
>  #ifndef __ASSEMBLY__
>
> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> index 0dbdb4f3634f..9c3f95f30421 100644
> --- a/arch/arm64/kernel/head.S
> +++ b/arch/arm64/kernel/head.S
> @@ -274,18 +274,21 @@ ENDPROC(preserve_boot_args)
>   *     virt:   virtual address
>   *     shift:  #imm page table shift
>   *     ptrs:   #imm pointers per table page
> + *     offset: #imm offset into the lowest translation level, in pages
>   *
>   * Preserves:  virt
>   * Corrupts:   tmp1, tmp2
>   * Returns:    tbl -> next level table page address
>   */
> -       .macro  create_table_entry, tbl, virt, shift, ptrs, tmp1, tmp2
> +       .macro  create_table_entry, tbl, virt, shift, ptrs, tmp1, tmp2, offset=0
>         lsr     \tmp1, \virt, #\shift
> +       .if     \offset
> +       add     \tmp1, \tmp1, #\offset
> +       .endif
>         and     \tmp1, \tmp1, #\ptrs - 1        // table index
> -       add     \tmp2, \tbl, #PAGE_SIZE
> +       add     \tmp2, \tbl, #(\offset + 1) * PAGE_SIZE
>         orr     \tmp2, \tmp2, #PMD_TYPE_TABLE   // address of next table and entry type
>         str     \tmp2, [\tbl, \tmp1, lsl #3]
> -       add     \tbl, \tbl, #PAGE_SIZE          // next level table page
>         .endm
>
>  /*
> @@ -297,9 +300,14 @@ ENDPROC(preserve_boot_args)
>   */
>         .macro  create_pgd_entry, tbl, virt, tmp1, tmp2
>         create_table_entry \tbl, \virt, PGDIR_SHIFT, PTRS_PER_PGD, \tmp1, \tmp2
> -#if SWAPPER_PGTABLE_LEVELS == 3
> +#if SWAPPER_PGTABLE_LEVELS != 3
> +       create_table_entry \tbl, \virt, PGDIR_SHIFT, PTRS_PER_PGD, \tmp1, \tmp2, 1
> +#else
> +       add     \tbl, \tbl, #PAGE_SIZE          // next level table page
>         create_table_entry \tbl, \virt, TABLE_SHIFT, PTRS_PER_PTE, \tmp1, \tmp2
> +       create_table_entry \tbl, \virt, TABLE_SHIFT, PTRS_PER_PTE, \tmp1, \tmp2, 1
>  #endif
> +       add     \tbl, \tbl, #PAGE_SIZE          // next level table page
>         .endm
>
>  /*
> @@ -396,6 +404,7 @@ __create_page_tables:
>         str_l   x5, idmap_t0sz, x6
>
>         create_table_entry x0, x3, EXTRA_SHIFT, EXTRA_PTRS, x5, x6
> +       add     x0, x0, #PAGE_SIZE              // next level table page
>  1:
>  #endif
>
> --
> 1.8.3.2
>