linux-kernel - Re: [PATCHv3 6/8] x86/mm: Provide ARCH_GET_UNTAG_MASK and ARCH_ENABLE_TAGGED

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Date:   Tue, 28 Jun 2022 16:42:40 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, Kostya Serebryany <kcc@...gle.com>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Andrey Konovalov <andreyknvl@...il.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: [PATCHv3 6/8] x86/mm: Provide ARCH_GET_UNTAG_MASK and
 ARCH_ENABLE_TAGGED_ADDR

On 6/10/22 07:35, Kirill A. Shutemov wrote:

> +	/* Update CR3 to get LAM active */
> +	switch_mm(current->mm, current->mm, current);

Can you at least justify this oddity?  When changing an LDT, we use a 
dedicated mechanism.  Is there a significant benefit to abusing 
switch_mm for this?

Also, why can't we enable LAM on a multithreaded process?  We can change 
an LDT, and the code isn't even particularly complicated.