[PATCH 2/2] arm64: choose memstart_addr based on minimum sparsemem section alignment

[Ard Biesheuvel]

On 21 March 2016 at 18:38, Ard Biesheuvel <ard.biesheuvel at linaro.org> wrote:
> This reverts commit 36e5cd6b897e, which was needed in v4.5 and before to
> ensure the correct alignment of the base of the vmemmap region. However,
> since commit a7f8de168ace ("arm64: allow kernel Image to be loaded anywhere
> in physical memory"), the alignment of memstart_addr itself can be freely
> chosen, which means we can choose it such that additional rounding in the
> definition of vmemmap is no longer necessary.
>
> So redefine ARM64_MEMSTART_ALIGN in terms of the minimal alignment required
> by sparsemem, and drop the redundant rounding in the definition of vmemmap.
>
> Note that the net result of this change is that we align memstart_addr to
> 1 GB in all cases, since sparsemem is mandatory on arm64.

This is not actually true, since the 1 GB alignment on 64k pages is
only used for sparsemem-vmemmap. Also, this patch is no longer a
straight revert of the late fix we sent for v4.5, so perhaps it is
more appropriate to split off the updated definition of
ARM64_MEMSTART_ALIGN, so that we can put the straight revert on top of
that?


>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
> ---
>  arch/arm64/include/asm/kernel-pgtable.h | 17 +++++++++++++++--
>  arch/arm64/include/asm/pgtable.h        |  5 ++---
>  2 files changed, 17 insertions(+), 5 deletions(-)
>
> diff --git a/arch/arm64/include/asm/kernel-pgtable.h b/arch/arm64/include/asm/kernel-pgtable.h
> index 5c6375d8528b..a144ae2953a2 100644
> --- a/arch/arm64/include/asm/kernel-pgtable.h
> +++ b/arch/arm64/include/asm/kernel-pgtable.h
> @@ -19,6 +19,7 @@
>  #ifndef __ASM_KERNEL_PGTABLE_H
>  #define __ASM_KERNEL_PGTABLE_H
>
> +#include <asm/sparsemem.h>
>
>  /*
>   * The linear mapping and the start of memory are both 2M aligned (per
> @@ -87,9 +88,21 @@
>   * in the page tables: 32 * PMD_SIZE (16k granule)
>   */
>  #ifdef CONFIG_ARM64_64K_PAGES
> -#define ARM64_MEMSTART_ALIGN   SZ_512M
> +#define ARM64_MEMSTART_BITS    29
>  #else
> -#define ARM64_MEMSTART_ALIGN   SZ_1G
> +#define ARM64_MEMSTART_BITS    30
> +#endif
> +
> +/*
> + * sparsemem imposes an additional requirement on the alignment of
> + * memstart_addr, due to the fact that the base of the vmemmap region
> + * has a direct correspondence, and needs to appear sufficiently aligned
> + * in the virtual address space.
> + */
> +#if defined(CONFIG_SPARSEMEM_VMEMMAP) && ARM64_MEMSTART_BITS < SECTION_SIZE_BITS
> +#define ARM64_MEMSTART_ALIGN   (1UL << SECTION_SIZE_BITS)
> +#else
> +#define ARM64_MEMSTART_ALIGN   (1UL << ARM64_MEMSTART_BITS)
>  #endif
>
>  #endif /* __ASM_KERNEL_PGTABLE_H */
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index 989fef16d461..aa6106ac050c 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -32,14 +32,13 @@
>   * VMALLOC_END: extends to the available space below vmmemmap, PCI I/O space,
>   *     fixed mappings and modules
>   */
> -#define VMEMMAP_SIZE           ALIGN((1UL << (VA_BITS - PAGE_SHIFT)) * sizeof(struct page), PUD_SIZE)
> +#define VMEMMAP_SIZE           ALIGN((1UL << (VA_BITS - PAGE_SHIFT - 1)) * sizeof(struct page), PUD_SIZE)
>
>  #define VMALLOC_START          (MODULES_END)
>  #define VMALLOC_END            (PAGE_OFFSET - PUD_SIZE - VMEMMAP_SIZE - SZ_64K)
>
>  #define VMEMMAP_START          (VMALLOC_END + SZ_64K)
> -#define vmemmap                        ((struct page *)VMEMMAP_START - \
> -                                SECTION_ALIGN_DOWN(memstart_addr >> PAGE_SHIFT))
> +#define vmemmap                        ((struct page *)VMEMMAP_START - (memstart_addr >> PAGE_SHIFT))
>
>  #define FIRST_USER_ADDRESS     0UL
>
> --
> 1.9.1
>