linux-kernel - Re: [PATCHv7 13/14] x86: Enable 5-level paging support

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Date:   Thu, 22 Jun 2017 11:24:54 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...capital.net>,
        linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv7 13/14] x86: Enable 5-level paging support


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

> Most of things are in place and we can enable support of 5-level paging.
> 
> The patch makes XEN_PV dependent on !X86_5LEVEL. XEN_PV is not ready to
> work with 5-level paging.

Please make a short comment about that in the Kconfig code as well, instead of a 
silent, undocumented 'depends' clause.

>  config PGTABLE_LEVELS
>  	int
> +	default 5 if X86_5LEVEL
>  	default 4 if X86_64
>  	default 3 if X86_PAE
>  	default 2
> @@ -1390,6 +1391,10 @@ config X86_PAE
>  	  has the cost of more pagetable lookup overhead, and also
>  	  consumes more pagetable space per process.
>  
> +config X86_5LEVEL
> +	bool "Enable 5-level page tables support"
> +	depends on X86_64

So since users will be enabling it, this needs a proper help text that explains 
what hardware supports it ("future Intel CPUs" will do if models are not public 
yet), a short blurb about what it's good for - and a link to the Documentation/ 
file explaining it all.

Thanks,

	Ingo