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Shutemov" Subject: RE: [RFC PATCH v11 12/29] KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory Thread-Topic: [RFC PATCH v11 12/29] KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory Thread-Index: AQHZudJw1MgLsjkkR0ChgAdWyO82ca/KHE2ggACTTICAAJT1MA== Date: Wed, 26 Jul 2023 01:51:05 +0000 Message-ID: References: <20230718234512.1690985-1-seanjc@google.com> <20230718234512.1690985-13-seanjc@google.com> In-Reply-To: Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-publictraffictype: Email x-ms-traffictypediagnostic: DS0PR11MB6373:EE_|IA0PR11MB7209:EE_ x-ms-office365-filtering-correlation-id: 1f741d1d-5183-4c78-e944-08db8d7ac65f 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; x-microsoft-antispam-message-info: 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 x-forefront-antispam-report: CIP:255.255.255.255;CTRY:;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:DS0PR11MB6373.namprd11.prod.outlook.com;PTR:;CAT:NONE;SFS:(13230028)(346002)(39860400002)(136003)(366004)(396003)(376002)(451199021)(6916009)(8676002)(8936002)(7416002)(7406005)(52536014)(5660300002)(41300700001)(316002)(4326008)(2906002)(54906003)(66946007)(66446008)(64756008)(76116006)(66476007)(66556008)(7696005)(9686003)(26005)(71200400001)(53546011)(478600001)(186003)(6506007)(82960400001)(86362001)(55016003)(83380400001)(38100700002)(38070700005)(33656002)(122000001);DIR:OUT;SFP:1102; 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> > > + struct kvm_gmem *gmem; > > > + struct folio *folio; > > > + struct page *page; > > > + struct file *file; > > > + > > > + file =3D kvm_gmem_get_file(slot); > > > + if (!file) > > > + return -EFAULT; > > > + > > > + gmem =3D file->private_data; > > > + > > > + if (WARN_ON_ONCE(xa_load(&gmem->bindings, index) !=3D slot)) { > > > + fput(file); > > > + return -EIO; > > > + } > > > + > > > + folio =3D kvm_gmem_get_folio(file_inode(file), index); > > > + if (!folio) { > > > + fput(file); > > > + return -ENOMEM; > > > + } > > > + > > > + page =3D folio_file_page(folio, index); > > > + > > > + *pfn =3D page_to_pfn(page); > > > + *max_order =3D compound_order(compound_head(page)); > > > > Maybe better to check if caller provided a buffer to get the max_order: > > if (max_order) > > *max_order =3D compound_order(compound_head(page)); > > > > This is what the previous version did (restrictedmem_get_page), so > > that callers who only want to get a pfn don't need to define an unused > > "order" param. >=20 > My preference would be to require @max_order. I can kinda sorta see why = a > generic implementation (restrictedmem) would make the param optional, but > with gmem being KVM-internal I think it makes sense to require the param. > Even if pKVM doesn't _currently_ need/want the order of the backing > allocation, presumably that's because hugepage support is still on the TO= DO > list, not because pKVM fundamentally doesn't need to know the order of th= e > backing allocation. Another usage is live migration. The migration flow works with 4KB pages on= ly, and we only need to get the pfn from the given gfn. "order" doesn't seem to= be useful for this case.