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Mon, 14 Jul 2025 23:09:32 +0000 Date: Mon, 14 Jul 2025 18:11:05 -0500 From: Ira Weiny To: Vishal Annapurve , Sean Christopherson CC: Ira Weiny , Michael Roth , "Yan Zhao" , , , , , , , , , , , , , , , , Subject: Re: [RFC PATCH] KVM: TDX: Decouple TDX init mem region from kvm_gmem_populate() Message-ID: <68758e893367f_38ba712947a@iweiny-mobl.notmuch> References: <20250703062641.3247-1-yan.y.zhao@intel.com> <20250709232103.zwmufocd3l7sqk7y@amd.com> <20250711151719.goee7eqti4xyhsqr@amd.com> <20250711163440.kwjebnzd7zeb4bxt@amd.com> <68717342cfafc_37c14b294a6@iweiny-mobl.notmuch> Content-Type: text/plain; charset="utf-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-ClientProxiedBy: MW4P221CA0028.NAMP221.PROD.OUTLOOK.COM (2603:10b6:303:8b::33) To PH3PPF9E162731D.namprd11.prod.outlook.com (2603:10b6:518:1::d3c) Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: PH3PPF9E162731D:EE_|IA1PR11MB7755:EE_ X-MS-Office365-Filtering-Correlation-Id: 7e035718-2d40-4499-574d-08ddc32b7e5a X-LD-Processed: 46c98d88-e344-4ed4-8496-4ed7712e255d,ExtAddr X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|7416014|376014|1800799024|366016; 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> > > > > > Why convert from private to shared and back to private? Userspace which > > > knows about mmap and supports it should create shared pages, mmap, write > > > data, then convert to private. > > > > Dunno if there's a strong usecase for converting to shared *and* populating the > > data, but I also don't know that it's worth going out of our way to prevent such > > behavior, at least not without a strong reason to do so. E.g. if it allowed for > > a cleaner implementation or better semantics, then by all means. But I don't > > think that's true here? Though I haven't thought hard about this, so don't > > quote me on that. :-) > > If this is a huge page backing, starting as shared will split all the > pages to 4K granularity upon allocation. Why? What is the reason it needs to be split? > To avoid splitting, userspace > can start with everything as private when working with hugepages and > then follow convert to shared -> populate -> convert to private as > needed. I'm not saying this could not be done but seems wasteful. Ira