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b=UY6kRCW5OLnNZGd6v5G3URHUnWGaqSALN0lwGq/2t8h5pPDKs/mbx7fxtxJ4BXM4tjXGKU7/N7cMkpdEQvgzIxsw6HJOanxGjbPscDN1Q5cdS1GKgBF+7gpz5e8edm69vEiVi6mrtNJ5axM/GG9FghGa0huc25ZIoGKQCaGx+Ng= 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 DM4PR12MB6327.namprd12.prod.outlook.com (2603:10b6:8:a2::9) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.21.223.10; Wed, 15 Jul 2026 14:09:42 +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 15.21.0202.014; Wed, 15 Jul 2026 14:09:40 +0000 Message-ID: <059088ac-81e7-48a9-b4e1-05dad48975f6@amd.com> Date: Wed, 15 Jul 2026 16:09:34 +0200 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] KVM: SEV: drop FOLL_WRITE for encrypted region registration To: "David Hildenbrand (Arm)" , seanjc@google.com, pbonzini@redhat.com, tglx@kernel.org, bp@alien8.de, mingo@redhat.com, dave.hansen@linux.intel.com Cc: 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> 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. > > 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. > > In general, LGTM > > Acked-by: David Hildenbrand (Arm) thanks Pankaj