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b=YwNxgp8npLzSUsNaGSIwqTZqRjfKxwoc8AeXFzfTN+BzpBD4p/+r8k0BqTJd1bWR3zfCzxd9EZh+4ye41arYEZNU5rr+wYOFvvTv7B6LE92AlVN7dB5Y7DsQJgRGPGK4bqCRSOKn0ZKDWkqFZO6DcW1PDTvUFWYPgZBI42nzL+M= Authentication-Results: dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=amd.com; Received: from DS0PR12MB7771.namprd12.prod.outlook.com (2603:10b6:8:138::6) by MW4PR12MB7141.namprd12.prod.outlook.com (2603:10b6:303:213::20) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.21.245.10; Tue, 21 Jul 2026 02:09:02 +0000 Received: from DS0PR12MB7771.namprd12.prod.outlook.com ([fe80::9a3e:791f:33b6:3d8c]) by DS0PR12MB7771.namprd12.prod.outlook.com ([fe80::9a3e:791f:33b6:3d8c%5]) with mapi id 15.21.0223.017; Tue, 21 Jul 2026 02:09:02 +0000 Message-ID: <001fa28e-0355-4f06-9110-d8ef44d963e3@amd.com> Date: Tue, 21 Jul 2026 04:08:57 +0200 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] KVM: SEV: drop FOLL_WRITE for encrypted region registration To: Sean Christopherson Cc: "David Hildenbrand (Arm)" , pbonzini@redhat.com, tglx@kernel.org, bp@alien8.de, mingo@redhat.com, dave.hansen@linux.intel.com, x86@kernel.org, thomas.lendacky@amd.com, hpa@zytor.com, yangge1116@126.com, ljs@kernel.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20260715063626.65899-1-pankaj.gupta@amd.com> <059088ac-81e7-48a9-b4e1-05dad48975f6@amd.com> Content-Language: en-US From: "Gupta, Pankaj" In-Reply-To: Content-Type: text/plain; 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It also kept >>>> FOLL_WRITE on the pin. >>>> >>>> Combining FOLL_WRITE with FOLL_LONGTERM breaks registration of file-backed >>>> guest memory, such as virtio-pmem host memory-backend-file mappings >>>> (MAP_SHARED). GUP rejects long-term writable pins on dirty tracked file >>>> mappings since: >>>> >>>> commit 8ac268436e6d ("mm/gup: disallow FOLL_LONGTERM GUP-nonfast writing to file-backed mappings") >>>> commit a6e79df92e4a ("mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings"). >>>> >>>> Region registration only requires long-term pin to prevent page migration and >>>> does not write through this GUP pin. >>>> >>>> Drop FOLL_WRITE and pin guest memory only with FOLL_LONGTERM. >>> Worth mentioning here something like >>> >>> "In the past, FOLL_WRITE was required to trigger CoW unsharing, making sure that >>> we don't end up replacing the page in the page tables during a later write fault >>> after already having pinned a (shared) page in MAP_PRIVATE mappings. >>> FOLL_LONGTERM does that nowadays (see gup_must_unshare()) even without FOLL_WRITE. > Heh, this was going to be my exact question about why it was safe to drop FOLL_WRITE :-) > >>> Given that SEV only pins RAM for XYZ and doesn't actually write to the pinned >>> pages, we can just drop the FOLL_WRITE" >>> >>> Fill out XYZ :) >> Sure :) >> >> I will update the commit message in v3. > No need for a v3, I'll add a blurb when applying. Thanks, Sean! Best regards, Pankaj