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Subject: Re: [PATCHv3 6/8] x86/mm: Provide ARCH_GET_UNTAG_MASK and
 ARCH_ENABLE_TAGGED_ADDR
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To:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     x86@kernel.org, Kostya Serebryany <kcc@google.com>,
        Andrey Ryabinin <ryabinin.a.a@gmail.com>,
        Andrey Konovalov <andreyknvl@gmail.com>,
        Alexander Potapenko <glider@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        "H . J . Lu" <hjl.tools@gmail.com>,
        Andi Kleen <ak@linux.intel.com>,
        Rick Edgecombe <rick.p.edgecombe@intel.com>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
References: <20220610143527.22974-1-kirill.shutemov@linux.intel.com>
 <20220610143527.22974-7-kirill.shutemov@linux.intel.com>
From:   Andy Lutomirski <luto@kernel.org>
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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.