From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D679A30C17D; Mon, 13 Jul 2026 16:05:11 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783958713; cv=none; b=KeqfhhtTYQXp+i5eAHq2jQZFXqxIYSJiVn0EdIs27i90Mww5TdFDsEKLNO+jaruJnm52+rRPQbPIUMW/P3e6wa0UNCzUThTAwaUONBrx/LWfwPXWBSHA1TIgo8uWcXyojoCxEFmq1it1ZjKYG2AqckJe59CfA61lMqn4SKAWHks= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783958713; c=relaxed/simple; bh=oibnUKSYXikDF+73wKpPOWQmjAAJwAwVq1TqJiuicvU=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=YeR16D/3TszwCadWsTmY9/muvkSjpFaiAmHfu8NEy5tyn7XOoo62ri+UOlk/Conv8BlQWSF3dhBlKAkwvo4ErVttPxieckEdJamt87ZXk5j3F9Rbw3dlWd/BmN8+hSp6bIgYKQgeQjEwtb+z4j1L3qXVIModSabGtRt8ZoPxuo8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=BvAfS+kz; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="BvAfS+kz" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 224DD1F000E9; Mon, 13 Jul 2026 16:04:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1783958710; bh=1ESM2SzSD3UynOgixO3ALqhK0K8aaLtWF+/LHsivKfc=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=BvAfS+kzALSX82y1Di2VtOQp38EMVYa/a0vHm/IjTpc7kf4jHjuOTM3sKo8R84O9c xOk4JPna7qvfy1PaukIZF3T6cPMAXfpfTSZDna4XE+oYrFfmsKMFc4d0xDKcU5/eKx U2YwGSZRSNFKcvP67THVBGvhPF76bHe3Vlejfo+krhMrjRfHJBFUB8U4uJ9SFX0fWQ Qts/DhSoySClKbyxe4FlftP3EjW4m8SF2SIajlKKylzaXj3KgTYmFpd3BWKFz00kdW b89lbYsbbXIQskkvAlEcQj7q/YxyAGkwddulKX1HhX8qMFejkF1bio8/BTAmHZMc7B eOM3fNCfKhXaw== Message-ID: <2858abeb-4134-4768-9a35-a569d5c86e10@kernel.org> Date: Mon, 13 Jul 2026 18:04:54 +0200 Precedence: bulk X-Mailing-List: linux-mips@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 02/34] ARM: mm: make 2-level pgd_t a scalar To: Arnd Bergmann , Yeoreum Yun , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, loongarch@lists.linux.dev, linux-mips@vger.kernel.org, Linux-Arch , kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org, x86@kernel.org, linux-mm@kvack.org, kasan-dev@googlegroups.com, "linux-csky@vger.kernel.org" , linux-m68k@lists.linux-m68k.org, "linux-openrisc@vger.kernel.org" Cc: Russell King , Andrew Morton , Ankur Arora , Mike Rapoport , Magnus Lindholm , Christophe Leroy , Klara Modin , Huacai Chen , WANG Xuerui , "Kirill A. Shutemov" , zhangtianyang@loongson.cn, wangyuli@aosc.io, Thomas Bogendoerfer , Lorenzo Stoakes , Jason Gunthorpe , Catalin Marinas , Will Deacon , Ryan Roberts , Pasha Tatashin , Rohan McLure , Baolin Wang , Tejun Heo , Kevin Brodsky , Anup Patel , atish.patra@linux.dev, Paul Walmsley , Palmer Dabbelt , Albert Ou , Alexandre Ghiti , Dave Hansen , Andy Lutomirski , Peter Zijlstra , Thomas Gleixner , Ingo Molnar , Borislav Petkov , "H. Peter Anvin" , Johannes Weiner , Michal Hocko , qi.zheng@linux.dev, Shakeel Butt , Kairui Song , Barry Song , Axel Rasmussen , Yuanchu Xie , Wei Xu , Andrey Ryabinin , Alexander Potapenko , Andrey Konovalov , Dmitry Vyukov , Vincenzo Frascino , Anshuman Khandual , Yang Shi , chaitanyas.prakash@arm.com, Ard Biesheuvel , guoren , yang.li85200@gmail.com, Alexander Viro , Dinh Nguyen , "schuster.simon@siemens-energy.com" , Vivian Wang , junhui.liu@pigmoral.tech, Muchun Song , "Vishal Moola (Oracle)" , Nam Cao , Pavel Machek , djbw@kernel.org, yu-cheng.yu@intel.com, Baolu Lu , Jonathan Cameron , Coiby Xu , Andreas Larsson , "Liam R. Howlett" , "Vlastimil Babka (SUSE)" , Suren Baghdasaryan , Michal Hocko , Geert Uytterhoeven , Stafford Horne , Jonas Bonn , Stefan Kristiansson References: <20260713135614.1618183-1-yeoreum.yun@arm.com> <20260713135614.1618183-3-yeoreum.yun@arm.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 7/13/26 17:38, Arnd Bergmann wrote: > On Mon, Jul 13, 2026, at 15:55, Yeoreum Yun wrote: >> From: "David Hildenbrand (Arm)" >> >> We don't want pgd_t to be an array, as it prohibits returning it from a >> function, like pgdp_get(). > > What should pgdp_get() return on ARM 2-stage page tables then? > Does it return just the first entry or concatenate the two? It is rather obscure what we do (below). This patch tries to keep the existing behavior. > >> +typedef u64 pgdval_t; > >> +static inline pmdval_t pgd_val(pgd_t pgd) >> +{ >> + return (*(pmdval_t (*)[2])&pgd)[0]; >> +} > > This could use a comment to explain what this actually > returns and why the second entry is discarded. It's rather crazy. We have two page table levels, but fake that we have 3. There is some documentation in arch/arm/include/asm/pgtable-2level.h /* * Hardware-wise, we have a two level page table structure, where the first * level has 4096 entries, and the second level has 256 entries. Each entry * is one 32-bit word. Most of the bits in the second level entry are used * by hardware, and there aren't any "accessed" and "dirty" bits. * * Linux on the other hand has a three level page table structure, which can * be wrapped to fit a two level page table structure easily - using the PGD * and PTE only. However, Linux also expects one "PTE" table per page, and * at least a "dirty" bit. * * Therefore, we tweak the implementation slightly - we tell Linux that we * have 2048 entries in the first level, each of which is 8 bytes (iow, two * hardware pointers to the second level.) The second level contains two * hardware PTE tables arranged contiguously, preceded by Linux versions * which contain the state information Linux needs. We, therefore, end up * with 512 entries in the "PTE" level. * * This leads to the page tables having the following layout: * * pgd pte * | | * +--------+ * | | +------------+ +0 * +- - - - + | Linux pt 0 | * | | +------------+ +1024 * +--------+ +0 | Linux pt 1 | * | |-----> +------------+ +2048 * +- - - - + +4 | h/w pt 0 | * | |-----> +------------+ +3072 * +--------+ +8 | h/w pt 1 | * | | +------------+ +4096 * So we interpret two 32bit entries as a pair (64bit value). Together they span 2x256 values. Then we make PTRS_PER_PTE = 512, to iterate all of them (both pairs). Let me do some more digging to come up with something I can confidentially document here ... maybe I'll just refer to the comment in pgtable-2level.h. -- Cheers, David