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Peter Anvin" , Peter Zijlstra , , , References: <20210422151007.2205-1-jgross@suse.com> <08e3fcf1-dabc-c550-f76c-47a78a12274b@suse.com> From: Andrew Cooper Subject: Re: [PATCH 0/3] xen: remove some checks for always present Xen features Message-ID: <51e880f0-698e-8208-1a04-fca3942cd6eb@citrix.com> Date: Thu, 22 Apr 2021 16:51:27 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Content-Language: en-GB X-ClientProxiedBy: LO2P265CA0416.GBRP265.PROD.OUTLOOK.COM (2603:10a6:600:a0::20) To BYAPR03MB3623.namprd03.prod.outlook.com (2603:10b6:a02:aa::12) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 X-MS-PublicTrafficType: Email X-MS-Office365-Filtering-Correlation-Id: 2ab3da4e-d4ba-4604-bf07-08d905a681d1 X-MS-TrafficTypeDiagnostic: SJ0PR03MB5454: X-Microsoft-Antispam-PRVS: X-MS-Oob-TLC-OOBClassifiers: OLM:10000; X-MS-Exchange-SenderADCheck: 1 X-Microsoft-Antispam: BCL:0; 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X-MS-Exchange-AntiSpam-MessageData: =?utf-8?B?VzEyTHlWdHJYeEhPaWNXUndkV3dLMG1FQy9Ra2FCZTZGcWtZU2pqdVlRelVB?= =?utf-8?B?eGcrZ2tJTVdpSXFzeVZCWXNGNU05TUpWVzU5V2FwN2hObVBIejBUamx3bmdr?= =?utf-8?B?VnlobWFSRjJjWXhzVHJKb2h1MlprZmY4TGJYN2hNb2lMR0hjbXBRUDNTR1p0?= =?utf-8?B?RmxVTmNYRWVRLy9BSkNGdnRkT3pyRGNSNFJTbklXZU5ERE5laDJOaC95N3BV?= =?utf-8?B?K2RSUXp6dk5LcFFhNTBQaVpNYXFmTS9iMTZVaXdEUEhTNThlN3hLYWRrRFZX?= =?utf-8?B?MGZMSEZWOENOd0pCblZRaE0zR1h4dmszUU1WRDQyc25SWlNqMjNMais4R2FP?= =?utf-8?B?OGthbytHUmlha2F5Rmp5ZC9JaHVvelNXQXE4QzNiR0hSQlk2NllVQUwrRU9a?= =?utf-8?B?cWxzY3RHbXpFVHlMVFByejFsU2k2QysxREZlN2U2emdNKytnNjdPMEpaQVRt?= =?utf-8?B?aHVkU0VtVkFTd1hTdjJra1htZ1ZCeVdKNHR2SmErNXlWeW81N1M1dm5jd0VS?= =?utf-8?B?ZERsQ1lkMzNYT3AyU1BDamU3TjFERzNWSWo3b1ZlbHFOZTk5K3ZmNVV1N0s5?= =?utf-8?B?THd6Ykx3L255cFhlWGgzNHBwcEdvWnNQelpCd05vc1BGcldDaCt0YmdnRFNH?= =?utf-8?B?cW9TbTdSZFgxVnJ1ampGSElKQkxzUC9XVERNcEZUTDlaVnlqcFM2TWQvWUF3?= =?utf-8?B?ckYxc1djZE96VDhrSzJTaEVUNG4zOHNKaWsybGlwa1Q4TENzY1lmQmdGczZT?= =?utf-8?B?c1gzMm9rbG9UVVY0RWJJTTgxVm1WNzRmSTZiR0VyM0Q3RnpLenhUdXNSbC9i?= =?utf-8?B?alp0YWo4c3c2dXN6bUFud2dvWld5Y2w3QXl4WDBHUEtqaXBTQ3Urc3VSYkt2?= =?utf-8?B?SHkwdG1Ec1JkK3ByNWhpNDdKTUxXMFZYUk5EVnBabitRQytpbWVZM2V6R09T?= =?utf-8?B?V2V0cVlJVXZQZ0FQUHZ1cnJvSGtFU2c1bnJSenpla0hmcmo3ZjRUTFd0c05C?= =?utf-8?B?K0QxVUhyOGlmNE1kcVIwcWYxVjFxWWp1Z3BEMzd2dG1lTnk0Qzc0MVVXNDlo?= =?utf-8?B?RVdnVEluanhpWDFhcjNPQTBZTVFlQzBKVitIQW1oMlQyVGg5RG9MbDFYL2pq?= =?utf-8?B?aGJFZmlJUUVQUDJFSmpEdTZGdjBoOVJGZUxVYis3dFF5Sk5SWHpRTFhYc09Q?= =?utf-8?B?QThOaVNUd2xHajA0YVhrYzIxcmE1aFptTTRucnduTkR2Umh4dFM5UTdSbzFa?= =?utf-8?B?eVgveU1FQUs0YzBrWS8raC80SjJxdVFQanE1dXV0dVdsem1TRytBQUdoUWtQ?= =?utf-8?B?Yk1OczdEYUNNcEoxZ3BjSythb255Mm5ieFVrWlhpTzZPK01NVXl6RnJrQUh0?= =?utf-8?B?TTc3QmxIa09PY0R4aFhMeEs2K3BGUHNGdm8yd25lcHAzNGtxaERzb0doMkVO?= =?utf-8?B?eG9xL0djVmhONlRlVjVjRTNRN25wb1pLYVZweGcyU3J6ZmIwY2poOWJEU0N2?= =?utf-8?B?US9tNW9zaW8ydmZacmk1ZGMvdkIrY05kOFdxNmFqVk9ycmFFTUppM1B3RHVF?= =?utf-8?B?WEI5OTkvK1NyNWpDQjlpVmJESFFiVyttYTc3NE13aGd2NFloSFNScEV2K2JJ?= =?utf-8?B?a0R5RkMrdnE5U0hwaDRET1lJZXBBOWg0ZnpIbE95NUJvb3Zoa2llay9xTFU0?= =?utf-8?B?RHgyQnVBUlUxSjdyUlk1WGlFVVdFRUdNVmhWWVhhM09GRnpIcWFYU0g3dlVM?= =?utf-8?Q?61QLexk0eOMj+jC+QGn2a9TDd1SMZ+QSCOwy2pf?= X-MS-Exchange-CrossTenant-Network-Message-Id: 2ab3da4e-d4ba-4604-bf07-08d905a681d1 X-MS-Exchange-CrossTenant-AuthSource: BYAPR03MB3623.namprd03.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 22 Apr 2021 15:51:35.2471 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 335836de-42ef-43a2-b145-348c2ee9ca5b X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: lectmM80XmDjVJ/lg4fUUwO4vsq+8etMPz8UGERYG2tVYo9GlSpImt2Oum5Vn2DqG+5PEqnsGS8P1rGLVJT/R1H1NkNcf5XyUtOB1o2Wxf8= X-MS-Exchange-Transport-CrossTenantHeadersStamped: SJ0PR03MB5454 X-OriginatorOrg: citrix.com On 22/04/2021 16:42, Jan Beulich wrote: > On 22.04.2021 17:28, Juergen Gross wrote: >> On 22.04.21 17:23, Jan Beulich wrote: >>> On 22.04.2021 17:17, Juergen Gross wrote: >>>> On 22.04.21 17:16, Jan Beulich wrote: >>>>> On 22.04.2021 17:10, Juergen Gross wrote: >>>>>> Some features of Xen can be assumed to be always present, so add a >>>>>> central check to verify this being true and remove the other checks. >>>>>> >>>>>> Juergen Gross (3): >>>>>> xen: check required Xen features >>>>>> xen: assume XENFEAT_mmu_pt_update_preserve_ad being set for pv g= uests >>>>>> xen: assume XENFEAT_gnttab_map_avail_bits being set for pv guest= s >>>>> I wonder whether it's a good idea to infer feature presence from >>>>> version numbers. If (at some point in the past) you had inferred >>>>> gnttab v2 being available by version, this would have been broken >>>>> by its availability becoming controllable by a command line option >>>>> in Xen. >>>> I'm testing the feature to be really present when booting and issue a >>>> message if it is not there. >>> And how does this help if the feature really isn't there yet other code >>> assumes it is? >> Did you look at the features I'm testing? > I did, yes. > >> Those are really just low >> level additions I can't imagine will ever be removed again. > I don't expect them to be removed. But I don't think the people having > contributed gnttab v2 expected any such for it, either. The trainwreck around gnttab v2 is a mistake I hope we're never going to make again.=C2=A0 I don't think it's a useful argument here. The logic is fine.=C2=A0 It's checking for the actual features in the ABI upon which Linux depends. Sure - someone could modify Xen to take the feature out, but they'd get a red wall in CI as they break every Linux kernel released in the past decade. ~Andrew