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a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789565540; bh=DKoMOvgofwktZmoUuUYwgSaKNRpTiD37gRdhxHcH9IM=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=bNk89v3Rdvd38fFvrhXtrco6p421SlhJ1qFm7IaLPOFUxKu/h9ghCRcc6TBl0FUuk pW+G8O/SLu4S/+mCwe2OGNjQSRU2vzMj9hu4UtooAkrxqP2dSN2cn5H48m33qMrL+D Yshy7d9e/IVpTidQ8mlVvmca86Od3soaTPQ54nJxtI6XF7I0RSAkuCsVS7Hgp+gbNx NQ8xPZFF3vSt+TF0bJnYpRuN2Gr2FKJ1uTPzwS5UG2CJjq5m4OiIGddUfm0dn0oN/V HZ1XMKE3+y6o6z/iD2WbXm8GPrr95SNcSbdHw/DF0gDeYQJTa0ysNz7EVh2t23BdXM wDAvvkmOWtTTQ== Message-ID: Date: Wed, 16 Sep 2026 15:32:14 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 02/17] arm64/mm: Convert READ_ONCE() as pmdp_get() while accessing PMD To: Anshuman Khandual , linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org Cc: Catalin Marinas , Will Deacon , Ryan Roberts , Mark Rutland , Lorenzo Stoakes , Andrew Morton , Mike Rapoport , Linu Cherian , linux-kernel@vger.kernel.org, Mark Rtland , linx-arm-kernel@lists.infradead.org, linx-kernel@vger.kernel.org, kasan-dev@googlegrops.com References: <20260729122452.3797443-1-anshuman.khandual@arm.com> <20260729122452.3797443-3-anshuman.khandual@arm.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: AC8BC1C000D X-Stat-Signature: bwhxi531feypao8rh9hyr1w43n4jp5zy X-HE-Tag: 1789565540-481938 X-HE-Meta: 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 efMdPd8B aQfEuSRaOJiADonBSwRTeiyHV7rpFhdYuL1wK6aZ5c5s9CIjPJPZZq/i1iXIFAlV1V6nfIRbS3MqZWQrZ+GzsXE6UEsUc/T1QzD+W3ER8JdPWCzfefy6QH5lv23zJdWJoRlx7pBz5iJO3/ku2J/Hj0IgTIjl3SbTVhq2tEVKMnkK3+vyJqYw4QXIcsDU46eKcTAjIGA+HZE6mhzbORFmdcByizrz8bjQ3kQyaQ5BjMsW78wI7EF6Ze/x7mP3QANvYpZ1TMYvmwFzZWkjNiyEtW6iKx+aK16mLU3wUpIwnILy+wl76NK3M2D/h60d1wtOSM0fu6b58orKz2Ow= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 7/29/26 14:24, Anshuman Khandual wrote: > Convert all READ_ONCE() based PMD accesses as pmdp_get() instead which will > support both D64 and D128 translation regime going forward. That is because > READ_ONCE() would need 128 bit single copy atomic guarantees, while reading > 128 bit page table entries which is currently not supported on arm64. Build > fails for READ_ONCE() while accessing beyond 64 bits. > > Load Pair/Store Pair (ldp/stp) are only single copy atomic if FEAT_LSE128 > is supported (which is required when FEAT_D128 is supported). Currently 128 > bit pgtables is a compile time decision - so we could have chosen to extend > READ_ONCE()/WRITE_ONCE() to allow 128 bit for this configuration. But then > it's a general purpose API and we were concerned that other users might > eventually creep in that expect 128 and then fail to compile in the other > configs. > > But worse, we are considering eventually making D128 a boot time option, at > which point we'd have to make READ_ONCE() always allow 128 bit at compile > time but then it might silently tear at runtime. > > So our preference is to standardize on these existing helpers, which we can > override in arm64 to give the 128 bit single copy guarantee when required. > Best to mention here that no functional change is expect because the common-cde pmdp_get() etc helpers do exactly that. Reviewed-by: David Hildenbrand (Arm) -- Cheers, David