linux-kernel - Re: [PATCHv7 09/19] x86/mm: Make MAX_PHYSADDR_BITS and MAX_PHYSMEM

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Date:   Thu, 28 Sep 2017 10:25:14 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...capital.net>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        Borislav Petkov <bp@...e.de>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv7 09/19] x86/mm: Make MAX_PHYSADDR_BITS and
 MAX_PHYSMEM_BITS dynamic


* Kirill A. Shutemov <kirill.shutemov@...ux.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@...ux.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