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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?IEkANHF4v/FIo64B8bW+QjCd0S+VLgfkauSEgmU3BslQGakiq8HX6jNm/u21?= =?us-ascii?Q?0bQe2XapbXbe1+1jMr5qq+oNw3ewDMiuWHmk9pAbVx5E2i2NKQMm4zg1PNlU?= =?us-ascii?Q?1xMi6OOosjOQ4GOQ83zyH7ZG9eXqGiM4VqsIa4rAwntAjPRREhhXJLDSGV+8?= =?us-ascii?Q?7IFEh1uy/dgB2GA6MSI2iVVrm6gIZ2cf3gIw7ph1KQMDjk0dDutYl4iuDSpC?= =?us-ascii?Q?1pR64d//HBbwc6CBK4mdQwlYeyIMpzHEuFFh11Cqwmy0EZQGQ7K73w7lEvnT?= =?us-ascii?Q?nUzS95dyS3lRRRjlWVubn1ZC+LI4do5IB4/Sq9FfQhZN4c3QJrJMuryMIOxy?= =?us-ascii?Q?VLXKY6DXEXtgoRt0pGrcQDOOwI54OowreL6x0ri09bJX3y7cm8OEaUK+yEuB?= =?us-ascii?Q?rA+XVMkXprZZlG+h4qKHSgNeLffvdakh7VR9R/wbms/3zpzIr0bhSLanEKe3?= =?us-ascii?Q?Y76L1E9QIdBDT2wZtSW1pIl/BYMYlany2yK7+q1dQwB+CGj3V1qugNUzhvqm?= =?us-ascii?Q?peJVLz39adZXMxqfpX5j5LIDXKmIGt01MRtf2CQwGy8APsQwe4Pag+srcsrE?= =?us-ascii?Q?Q+QnUAjUpKcABZx2gTrTl9RxdaxY1gaPUyVeztonUKfujWX4a1gOKAi+tc22?= =?us-ascii?Q?3/l6qtwBJLG4mU2Jcwmrcx2skdxDgjnZ52FRLS+ANKlFoF7pLVxGypaj5eyE?= =?us-ascii?Q?X/53dmZJqORB1JXk6Psqe3uuYb6ZsJa+KwpjxJtpRykC8w38/6MkfdeH92J7?= =?us-ascii?Q?KkBnLm7VehgfxXFFaLNGiCYXNfZ6LNw/gF8njeITlmzWYBGd2qCY41iRzRGA?= =?us-ascii?Q?+RlLs8L/fQJdU90SxbImAegtV+8btUrOXnNqcytFLhbpdfq/5EDbHU/80Str?= =?us-ascii?Q?d+U0P7uYWWH0/pVCDJJ+qi1yfgJ8yzBUS4Kq2XpvjHMTrMyWqfCXbR/v3Qfv?= =?us-ascii?Q?xMH3HNLFoRZs/QKD2bADiLc//736oOJkXLQloAkBZKviXvnQZCp8okr+gE5x?= =?us-ascii?Q?ZKP2NDmt6hiI8ZDWrOrWDJ2SFn6GnTaDecIjlUGlN2vFBb7C3VhLR3LI4gnj?= =?us-ascii?Q?CLEuj7OH/hUQK6eg52uCWQ97/xJXdrEnxp6qpPMNhXkEIMK7MrB140VTbi0o?= =?us-ascii?Q?cqaFO2B6VPCtDYLfrNKG9jpW7IwV9O24It+AHejWIOsTx3pxWiBkheXBJhX4?= =?us-ascii?Q?hMJQDd8okfu+Oqs0DsQyNx7zZNh31fGxfeNGR/YFfe7xyho1srUZDzmqik9x?= =?us-ascii?Q?4w+OntDvtLJcDw/eH94TWjzHWFmEtab8+vhCtWubUlIgxQAEYzzQe9C9iFUH?= =?us-ascii?Q?zH+L+Bv2OPDzGVkouGxR7uxKONlz6vXXLF0FAwwyELsrj+zLLqwUoeY+8iGy?= =?us-ascii?Q?VSPpN62j/uSDqMS8V1xTg8eMOqVjAXaJUytvJuWZb0gLARhD6tqYaSq79Tn6?= =?us-ascii?Q?2FdcHT3R5cZTDjJdb7vFTy8IcAix4wIfE2b9N20EEMDnYXmntSAEt6BuDHbv?= =?us-ascii?Q?k/tgD2dgZIBcipXAV8lTGsyJHSTxz8kc9AZFSmK7lpNioi00Iue3s1ON5tev?= =?us-ascii?Q?+Qqjbd+q1jV/kOSiYVYyLLNSmWybVh/mw9cG4OQK?= X-MS-Exchange-CrossTenant-Network-Message-Id: 63b5ccdb-2867-4200-969d-08dd6631060e X-MS-Exchange-CrossTenant-AuthSource: CH3PR11MB8660.namprd11.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 18 Mar 2025 15:24:50.1881 (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: HAdyoj07p94cH1wtvVBOAVWvFDcTbFWN3HyiL5TPKYQcs1hEwl9TUexdXkUuo4vuU42bduGCAA6u5Q8Mr+b0Ig== X-MS-Exchange-Transport-CrossTenantHeadersStamped: IA1PR11MB8098 X-OriginatorOrg: intel.com On Fri, Mar 07, 2025 at 11:09:42AM -0800, Dave Hansen wrote: >On 3/7/25 08:41, Chao Gao wrote: >> case |IA32_XSS[12] | Space | RFBM[12] | Drop% >> -----+-------------+-------+----------+------ >> 1 | 0 | None | 0 | 0.0% >> 2 | 1 | None | 0 | 0.2% >> 3 | 1 | 24B? | 1 | 0.2% > >So, 0.2% is still, what, dozens of cycles? Are you sure that it really >takes the CPU dozens of cycles to skip over the feature during XSAVE? > >If it really turns out to be this measurable, we should probably follow >up with the folks that implement XSAVE and see what's going on under the >covers. I reran the performance tests and observed a run-to-run variation of 0.4% to 0.7%. So, I don't think there is any measurable performance difference. I will update the performance statements in the cover letter. > >On a separate note, I was bugging Thomas a bit on IRC. His memory was >that the AMX-era FPU rework only expected KVM to support user features. >You might want to dig through the history a bit and see if _that_ was >ever properly addressed because that would change the problem you're >trying to solve. I went through the email discussions and found only one relevant thread: https://lore.kernel.org/kvm/87wnmf66m5.ffs@tglx/#t where Thomas mentioned that guest_perm would be set as follows: guest_fpu::__state_perm = supported_xcr0 & xstate_get_group_perm(); If implemented this way, KVM would only support user features. However, the committed change is: 980fe2fddcff ("x86/fpu: Extend fpu_xstate_prctl() with guest permissions") In this change, fpu->guest_perm is copied from fpu->perm: + /* Same defaults for guests */ + fpu->guest_perm = fpu->perm; There are indeed some issues with enabling supervisor features for guest FPUs, but they have been addressed by recent changes in the tip-tree ([1], [2]) and patch 1 of this series. [1]: https://lore.kernel.org/all/20250218141045.85201-1-stanspas@amazon.de/ [2]: https://lore.kernel.org/all/20250317140613.1761633-1-chao.gao@intel.com/