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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: =?utf-8?B?aXhqTkNYWSt6UG5SejRaUDVNeWR4a04rZW5ZWHU4MFY0aVJXMWNqU2poajRs?= =?utf-8?B?enRCT1B6SHdBM3VaQ054bkZMRTcvNGlsWldOczFRQVVtQ2ErcGQyRU1UTGFQ?= =?utf-8?B?c0JOR0trUjNRV2JPSUVoUEVTazNrYkNsdVpMV0hNc0oxTzJ0cjJ5MERqUUlY?= =?utf-8?B?ekMzMk53aStIS1NibHdTRHRPSUN6cGJyWWkycytqVFR1UzNZSHJVdmpISlBo?= =?utf-8?B?REZ0M05pdTNFR25XMU5wRExFOEgvUGZyTFlmam1XRENlOVE4L1NuNUVrdzlo?= =?utf-8?B?RHNLaTBPUVVCUGV6SkM1WmY2YktHYzhJRTR6Mm1vQndOT3IrV0FGZGxtYy9G?= =?utf-8?B?UFozMlY4bUZyWDlzMUJKSGpYVUNQRjlQeFlzeW5qSXhaOVdHWVd5RTBVYk42?= =?utf-8?B?RFUwQ0NEclB4TU55enExWDN0Vjk0MTd6T0FTUkxwNjdqSGc2SmpBQVovdkpw?= =?utf-8?B?RjlYWGdCMVczYUN2R0hyM3VhcHVoWk5pZGpQa2Q5MU1heHRDdHdEN2xQZi95?= =?utf-8?B?U0daK0VQVVNXMHExRHJUVXUwbHEvQTBIYU9MaUhuN3lqZmZ2cityd2FkVXBM?= =?utf-8?B?eDVwbHhqSDFjTC9jaFVULzUxalg0NXhLUXZPS1VpT2sxTEZLU3R2RHhSRjdk?= =?utf-8?B?RzI3dWhnOUJnQWcvbFl4dVpjRjBGVUp0MUwreFNZM1JPTitVbk9pSlNQbmY4?= =?utf-8?B?eTUveHoxc0pwd0JQaVRVeksyeDZDUVpQU1NTYUNXUTJ4YU9LQ3l0Mk1IVFFX?= =?utf-8?B?VFVOTFlTemVKc0MyNDVTeTRENjYzRk1TUEpaNlVrWW1nWFpKNkxqUGVwZXpi?= =?utf-8?B?ZDQySlptcE82T2hoSmEzNjlVN1Y0TXdFcTZkL3RRZFhTSHpaRXFuRm1nZWs0?= =?utf-8?B?Y2s1V0FDZEpkbkNmRDFucWRwUnhZRG9GcVVvazZBTkNNNS96QWxEckllZ3Ri?= =?utf-8?B?eUtUUlBmUndhbkZ5YkhvSXFNYk1NcEtUQzFoczFzUU9kcTMvZnFTbTBYVXY0?= =?utf-8?B?NU1Da0N6RnduaElxbVIyU1ppamFmdHB4dS81M0pPUEdOODk4Y3FveHFzdXMx?= =?utf-8?B?TU9qVjBUc09FamxqK3pyRmYrN25WUnl0Sk1GNktBTDBFQUFWcWlOVUpUTDJT?= =?utf-8?B?bXpRMk83QWltTGlKZGpMN2pvNVoxWHJJUXRZQzhUblVSdWFCUFBsWm9ib3lE?= =?utf-8?B?YzYxWVpZN1lqTlhPNXZybzVLVkh0MzM4S1NBeDJxU0FSRkpzUXFmcHFON0Zv?= =?utf-8?B?dEpET3Jrdk5pWnBMcnJhYldKNTIyakNCdHNPOGxURnZ6N092bktEc2h2TzZN?= =?utf-8?B?MTdjTnJESU1XUkhlWXloS1JLWjl6U3Rpc1RqVkVGM1lBdEVJdDNmTHNqdldx?= =?utf-8?B?MitIMHNhRkl6VW4zWDRGRlFJMnh0SlEvQ3ZudWVVQnZjOVVndEpqOVNSM0dF?= =?utf-8?B?aWdvREtZRmdQQzJzWlpmRzUzQ3dYOFgxcjd1VzZFM3RxeWpBWHJHK2lMb3c0?= =?utf-8?B?VDVwdW1nTUpLWHJQS0dCS0VtYVNGWjljS29KMDdlekw1K2Y2RW02Zk5qZ0pn?= =?utf-8?B?TnR5NTZiSm4xZXZFUG02N1F0OFBGdEtINzFHLzAyZUFoeWp3c0RjVnlkaTFN?= =?utf-8?B?STBJbkNUbE5ETFZvc0FncGI5NGk5enJBNGFwOXJYSEh6WGsvK040NVoyV0pv?= =?utf-8?B?cTljLy9JY01PT3JXRUN2c2paMGlPdStRdG9OaWxJeFFJT0RLV3Y3ZU0vTkdu?= =?utf-8?B?MldkT0RhdXhoQzlkQms0clluSTMvRDR2WkRUbmd4MkhYU3l4QTlWVWJndExU?= =?utf-8?B?V25mODdib0NmZ3pNeXBOV2JOZzJhMDhrSzc5QzlyemoycEZoV21WTCtVUEdZ?= =?utf-8?B?dWNSdFBiL3R0dUVLOVVkRTM1NEw5cE5MdjhCZGIxeHRoMmNmSHlSTDlRSFl4?= =?utf-8?B?cWZFWndTVmN1MGNkK0c1K3JYVWtHb3AxKzZmOVorUWNsb0VuSk0ycGFhc2dk?= =?utf-8?B?UzZHSThWRkFYQWJ2T3N3Rm9JV1JkelVwVU8vVUFJbC9lUC9vNms4TkJNNEs0?= =?utf-8?B?c3dBbUk5OTJ2cEJ4ZzEycEt5VTUrdm5rc1MyeFZNZ2trc25oZUxJVnIweDF6?= =?utf-8?B?UFBPMTZiVkpKK3ZhK2hYOG1hZm9jMUxTcnhodDdhcmNQYkhlRXMvcjM3WUF4?= =?utf-8?Q?hg7TiU+/vjCXPiMfTaSbV7k=3D?= X-MS-Exchange-CrossTenant-Network-Message-Id: 7d7ae954-7493-46a8-c739-08d9bef6a578 X-MS-Exchange-CrossTenant-AuthSource: DS7PR03MB5608.namprd03.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 14 Dec 2021 11:41:19.9274 (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: foxtRQdvv4cDmkI+zPXP2SZt04MeMYPmPpqP/6YUAIwMCmkaZHRgARf2X7i85HBYZEWlJQAgDEgp/DYy8F7qiw== X-MS-Exchange-Transport-CrossTenantHeadersStamped: DM6PR03MB5083 X-OriginatorOrg: citrix.com On Tue, Dec 14, 2021 at 10:15:37AM +0100, Jan Beulich wrote: > On 13.12.2021 16:51, Roger Pau Monné wrote: > > On Fri, Sep 24, 2021 at 11:54:58AM +0200, Jan Beulich wrote: > >> Page table are used for two purposes after allocation: They either start > >> out all empty, or they get filled to replace a superpage. Subsequently, > >> to replace all empty or fully contiguous page tables, contiguous sub- > >> regions will be recorded within individual page tables. Install the > >> initial set of markers immediately after allocation. Make sure to retain > >> these markers when further populating a page table in preparation for it > >> to replace a superpage. > >> > >> The markers are simply 4-bit fields holding the order value of > >> contiguous entries. To demonstrate this, if a page table had just 16 > >> entries, this would be the initial (fully contiguous) set of markers: > >> > >> index 0 1 2 3 4 5 6 7 8 9 A B C D E F > >> marker 4 0 1 0 2 0 1 0 3 0 1 0 2 0 1 0 Could you expand bit more on this explanation? I don't get how such markers are used, or how they relate to the page table entries. I guess the point is to note whether entries are populated, and whether such populated entries are contiguous? Could you provide a more visual example maybe, about what would go into each relevant page table entry, including the marker information? I would like to understand this instead of trying to figure out from the code (as then I could be making wrong assumptions). > >> > >> "Contiguous" here means not only present entries with successively > >> increasing MFNs, each one suitably aligned for its slot, but also a > >> respective number of all non-present entries. > > > > I'm afraid I'm slightly lost with all this, please bear with me. Is > > this just a performance improvement when doing super-page > > replacements, or there's more to it? > > What I wanted to strictly avoid is to have to scan entire pages for > contiguity (i.e. on average touching half a page), like e.g. > map_pages_to_xen() and modify_xen_mappings() do. Hence I tried to > find a scheme where for any individual update only a predictably > very limited number of entries need inspecting (some of these then > of course also need updating). Thanks. So there's some extra cost here of having to update those markers when a page table entry is modified. Roger.