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

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Date:   Sat, 14 May 2022 10:37:55 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, 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
Subject: Re: [RFCv2 08/10] x86/mm: Make LAM_U48 and mappings above 47-bits
 mutually exclusive

On Sat, May 14 2022 at 02:22, Kirill A. Shutemov wrote:

> On Thu, May 12, 2022 at 03:36:31PM +0200, Thomas Gleixner wrote:
>> 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.
>
> It is in 09/10. See lam_u48_allowed()

Sure, but that makes this changelog not any more correct.