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SubjectRe: [PATCHv7 09/19] x86/mm: Make MAX_PHYSADDR_BITS and MAX_PHYSMEM_BITS dynamic

* Kirill A. Shutemov <kirill.shutemov@linux.intel.com> wrote:

> For boot-time switching between paging modes, we need to be able to
> adjust size of physical address space at runtime.
>
> As part of making physical address space size variable, we have to make
> X86_5LEVEL dependent on SPARSEMEM_VMEMMAP. !SPARSEMEM_VMEMMAP
> configuration doesn't work well with variable MAX_PHYSMEM_BITS.
>
> Affect on kernel image size:
>
> text data bss dec hex filename
> 10710340 4880000 860160 16450500 fb03c4 vmlinux.before
> 10710666 4880000 860160 16450826 fb050a vmlinux.after
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
> arch/x86/Kconfig | 1 +
> arch/x86/include/asm/pgtable_64_types.h | 2 +-
> arch/x86/include/asm/sparsemem.h | 9 ++-------
> arch/x86/kernel/setup.c | 5 ++---
> 4 files changed, 6 insertions(+), 11 deletions(-)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 6a15297140ff..f75723d62c25 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1403,6 +1403,7 @@ config X86_PAE
> config X86_5LEVEL
> bool "Enable 5-level page tables support"
> depends on X86_64
> + depends on SPARSEMEM_VMEMMAP

Adding a 'depends on' to random kernel internal implementational details, to
support new hardware, sucks as an UI, as it will just randomly hide/show the new
hardware option if certain magic Kconfig combinations are set.

Please check how other architectures are doing it. (Hint: they are using select.)

Also, what is the real dependency here? Why don't the other memory models work,
what's the failure mode - won't build, won't boot, or misbehaves in some other
way?

Thanks,

Ingo

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