[04/16] x86/boot: Increase boot page table size

Previous calculations ignored pages implicitly mapped by ACPI code,
so theoretical upper limit is higher than was set.

Using 4KB pages is desirable for better memory protection granularity.
Approximately twice as much memory is required for those.

Increase initial page table size to 64 4KB page tables.

Signed-off-by: Evgeniy Baskov <baskov@ispras.ru>
---
 arch/x86/include/asm/boot.h | 26 ++++++++++++++------------
 1 file changed, 14 insertions(+), 12 deletions(-)

On Tue, 6 Sept 2022 at 12:41, Evgeniy Baskov <baskov@ispras.ru> wrote:
>
> Previous calculations ignored pages implicitly mapped by ACPI code,

I'm not sure I understand what this means. Which ACPI code and which
pages does it map?

> so theoretical upper limit is higher than was set.
>
> Using 4KB pages is desirable for better memory protection granularity.
> Approximately twice as much memory is required for those.
>
> Increase initial page table size to 64 4KB page tables.
>
> Signed-off-by: Evgeniy Baskov <baskov@ispras.ru>
> ---
>  arch/x86/include/asm/boot.h | 26 ++++++++++++++------------
>  1 file changed, 14 insertions(+), 12 deletions(-)
>
> diff --git a/arch/x86/include/asm/boot.h b/arch/x86/include/asm/boot.h
> index 9191280d9ea3..024d972c248e 100644
> --- a/arch/x86/include/asm/boot.h
> +++ b/arch/x86/include/asm/boot.h
> @@ -41,22 +41,24 @@
>  # define BOOT_STACK_SIZE       0x4000
>
>  # define BOOT_INIT_PGT_SIZE    (6*4096)
> -# ifdef CONFIG_RANDOMIZE_BASE
>  /*
>   * Assuming all cross the 512GB boundary:
>   * 1 page for level4
> - * (2+2)*4 pages for kernel, param, cmd_line, and randomized kernel
> - * 2 pages for first 2M (video RAM: CONFIG_X86_VERBOSE_BOOTUP).
> - * Total is 19 pages.
> + * (3+3)*2 pages for param and cmd_line
> + * (2+2+S)*2 pages for kernel and randomized kernel, where S is total number
> + *     of sections of kernel. Explanation: 2+2 are upper level page tables.
> + *     We can have only S unaligned parts of section: 1 at the end of the kernel
> + *     and (S-1) at the section borders. The start address of the kernel is
> + *     aligned, so an extra page table. There are at most S=6 sections in
> + *     vmlinux ELF image.
> + * 3 pages for first 2M (video RAM: CONFIG_X86_VERBOSE_BOOTUP).
> + * Total is 36 pages.
> + *
> + * Some pages are also required for UEFI memory map and
> + * ACPI table mappings, so we need to add extra space.
> + * FIXME: Figure out exact amount of pages.
>   */
> -#  ifdef CONFIG_X86_VERBOSE_BOOTUP
> -#   define BOOT_PGT_SIZE       (19*4096)
> -#  else /* !CONFIG_X86_VERBOSE_BOOTUP */
> -#   define BOOT_PGT_SIZE       (17*4096)
> -#  endif
> -# else /* !CONFIG_RANDOMIZE_BASE */
> -#  define BOOT_PGT_SIZE                BOOT_INIT_PGT_SIZE
> -# endif
> +# define BOOT_PGT_SIZE         (64*4096)
>
>  #else /* !CONFIG_X86_64 */
>  # define BOOT_STACK_SIZE       0x1000
> --
> 2.35.1
>

On 2022-10-19 10:08, Ard Biesheuvel wrote:
> On Tue, 6 Sept 2022 at 12:41, Evgeniy Baskov <baskov@ispras.ru> wrote:
>> 
>> Previous calculations ignored pages implicitly mapped by ACPI code,
> 
> I'm not sure I understand what this means. Which ACPI code and which
> pages does it map?

Code from boot/compressed/{acpi.c,efi.c} that touches ACPI/EFI tables
is currently mapping pages that contain the tables implicitly by
causing page faults. And those mappings may require additional
memory for page tables. It became more apparent when I were removing
memory mapping from page fault handler.

> 
>> so theoretical upper limit is higher than was set.
...