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Wed, 13 Aug 2025 17:10:26 +0000 Date: Wed, 13 Aug 2025 12:11:55 -0500 From: Ira Weiny To: Fuad Tabba , Michael Roth CC: Vishal Annapurve , Ackerley Tng , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , Subject: Re: [RFC PATCH v2 02/51] KVM: guest_memfd: Introduce and use shareability to guard faulting Message-ID: <689cc75bee26e_20a6d929463@iweiny-mobl.notmuch> References: <20250529054227.hh2f4jmyqf6igd3i@amd.com> <20250702232517.k2nqwggxfpfp3yym@amd.com> <20250703041210.uc4ygp4clqw2h6yd@amd.com> Content-Type: text/plain; charset="utf-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-ClientProxiedBy: MW4PR03CA0166.namprd03.prod.outlook.com (2603:10b6:303:8d::21) To PH3PPF9E162731D.namprd11.prod.outlook.com (2603:10b6:518:1::d3c) Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: PH3PPF9E162731D:EE_|DM3PPF1939049CF:EE_ X-MS-Office365-Filtering-Correlation-Id: 0480a450-b799-424e-4084-08ddda8c4c10 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|366016|1800799024; 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Are there any other reasons? > > > > > > > > > > > > Otherwise, maybe just settle on SHARED as a documented default (since at > > > > > > least non-CoCo VMs would be able to reliably benefit) and let > > > > > > CoCo/GUEST_MEMFD_FLAG_SUPPORT_SHARED VMs set PRIVATE at whatever > > > > > > granularity makes sense for the architecture/guest configuration. > > > > > > > > > > > > > > > > Because shared pages are split once any memory is allocated, having a > > > > > way to INIT_PRIVATE could avoid the split and then merge on > > > > > conversion. I feel that is enough value to have this config flag, what > > > > > do you think? > > > > > > > > > > I guess we could also have userspace be careful not to do any allocation > > > > > before converting. > > > > (Re-visiting this with the assumption that we *don't* intend to use mmap() to > > populate memory (in which case you can pretty much ignore my previous > > response)) > > > > I'm still not sure where the INIT_PRIVATE flag comes into play. For SNP, > > userspace already defaults to marking everything private pretty close to > > guest_memfd creation time, so the potential for allocations to occur > > in-between seems small, but worth confirming. > > > > But I know in the past there was a desire to ensure TDX/SNP could > > support pre-allocating guest_memfd memory (and even pre-faulting via > > KVM_PRE_FAULT_MEMORY), but I think that could still work right? The > > fallocate() handling could still avoid the split if the whole hugepage > > is private, though there is a bit more potential for that fallocate() > > to happen before userspace does the "manually" shared->private > > conversion. I'll double-check on that aspect, but otherwise, is there > > still any other need for it? > > It's not just about performance. I think that the need is more a > matter of having a consistent API with the hypervisors guest_memfd is > going to support. Memory in guest_memfd is shared by default, but in > pKVM for example, it's private by default. Therefore, it would be good ^^^^^^^^^^^^^^^^ And Coco's as well right? Ira > to have a way to ensure that all guest_memfd allocations can be made > private from the get-go. > > Cheers, > /fuad > [snip]