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charset="us-ascii" Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-AuthSource: BN9PR11MB5276.namprd11.prod.outlook.com X-MS-Exchange-CrossTenant-Network-Message-Id: e0cffb3b-f5cf-4fa3-b5cc-08d9ca71806d X-MS-Exchange-CrossTenant-originalarrivaltime: 29 Dec 2021 02:18:28.3160 (UTC) X-MS-Exchange-CrossTenant-fromentityheader: Hosted X-MS-Exchange-CrossTenant-id: 46c98d88-e344-4ed4-8496-4ed7712e255d X-MS-Exchange-CrossTenant-mailboxtype: HOSTED X-MS-Exchange-CrossTenant-userprincipalname: y/2Qqsy/eYI7Y5UYG2r3jjW9GjKWCsCsexB1AMUkXk4FZEsLfjEVNGe1rHZdoMSQhm9Mfh+BgbcKxPKcBVasoA== X-MS-Exchange-Transport-CrossTenantHeadersStamped: BN7PR11MB2706 X-OriginatorOrg: intel.com Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org > From: Sean Christopherson > Sent: Wednesday, December 29, 2021 7:39 AM >=20 > On Wed, Dec 22, 2021, Jing Liu wrote: > > Guest xstate permissions should be set by userspace VMM before vcpu > > creation. Extend KVM_SET_CPUID2 to verify that every feature reported > > in CPUID[0xD] has proper permission set. >=20 > Why? Nothing in the changelog, code, or comments explains why KVM > _needs_ to prevent > userspace from advertising bogus features to the guest. E.g. the virtual > address > width check exists because _KVM_ will do the wrong thing given a width > other than 48 > or 57, and explicity says as much in a comment. Advertising a known bogus feature due to lacking of permission does no good compared to failing it early even before the guest is running. This also avoids tons of complexity at run-time to deal with permission violation (e.g. introducing new kvm exit reason if you tracked the v2 discussion). But yes, we should add a clear comment here. Thanks Kevin