linux-kernel - Re: [RFCv2 08/10] x86/mm: Make LAM_U48 and mappings above 47-bits mutually exclusive

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Message-ID: <875ymax480.ffs@tglx>
Date:   Thu, 12 May 2022 15:36:31 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        "H . J . Lu" <hjl.tools@...il.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [RFCv2 08/10] x86/mm: Make LAM_U48 and mappings above 47-bits
 mutually exclusive

On Wed, May 11 2022 at 05:27, Kirill A. Shutemov wrote:
> LAM_U48 steals bits above 47-bit for tags and makes it impossible for
> userspace to use full address space on 5-level paging machine.

> Make these features mutually exclusive: whichever gets enabled first
> blocks the othe one.

So this patch prevents a mapping above 47bit when LAM48 is enabled, but
I fail to spot how an already existing mapping above 47bit would prevent
LAM48 from being enabled.

Maybe I'm missing something which makes this magically mutually
exclusive.

Thanks,

        tglx