linux-kernel - Re: [PATCHv1, RFC 0/8] Boot-time switching between 4- and 5-level paging

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Date:   Thu, 25 May 2017 16:24:24 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "the arch/x86 maintainers" <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-arch@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHv1, RFC 0/8] Boot-time switching between 4- and 5-level paging

On Thu, May 25, 2017 at 1:33 PM, Kirill A. Shutemov
<kirill.shutemov@...ux.intel.com> wrote:
> Here' my first attempt to bring boot-time between 4- and 5-level paging.
> It looks not too terrible to me. I've expected it to be worse.

If I read this right, you just made it a global on/off thing.

May I suggest possibly a different model entirely? Can you make it a
per-mm flag instead?

And then we

 (a) make all kthreads use the 4-level page tables

 (b) which means that all the init code uses the 4-level page tables

 (c) which means that all those checks for "start_secondary" etc can
just go away, because those all run with 4-level page tables.

Or is it just much too expensive to switch between 4-level and 5-level
paging at run-time?

              Linus