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Mon, 21 Sep 2026 17:03:50 +0000 Received: from DS4PPF691668CDD.namprd11.prod.outlook.com ([fe80::5a5c:9c7a:cd1d:d687]) by DS4PPF691668CDD.namprd11.prod.outlook.com ([fe80::5a5c:9c7a:cd1d:d687%6]) with mapi id 15.21.0428.015; Mon, 21 Sep 2026 17:03:49 +0000 Message-ID: <0cf55a0a-4bfb-46c5-8718-410ecc860e96@intel.com> Date: Mon, 21 Sep 2026 22:33:42 +0530 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 17/24] drm/xe: Add device flag to enable PT mirroring across tiles To: , Matthew Brost CC: References: <20260904211613.3934307-1-matthew.brost@intel.com> <20260904211613.3934307-18-matthew.brost@intel.com> <20260904214039.938631F00A3D@smtp.kernel.org> Content-Language: en-US From: "Ghimiray, Himal Prasad" In-Reply-To: <20260904214039.938631F00A3D@smtp.kernel.org> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: PN2PR01CA0183.INDPRD01.PROD.OUTLOOK.COM (2603:1096:c01:e8::9) To DS4PPF691668CDD.namprd11.prod.outlook.com (2603:10b6:f:fc02::2a) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS4PPF691668CDD:EE_|CO1PR11MB4803:EE_ X-MS-Office365-Filtering-Correlation-Id: 60b738a9-7d8a-4fa4-cc3a-08df18024e80 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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Sashiko AI review found 1 potential issue(s) to consider: > - [Medium] Unnecessary allocation of page table roots and scratch structures for secondary tiles when PT mirroring is disabled. Looks valid. > -- > > commit 34b539e899a7643dc15dedad6a3cd229e0c0f4d5 > Author: Matthew Brost > > drm/xe: Add device flag to enable PT mirroring across tiles > > This commit introduces a device flag to toggle page-table mirroring across > tiles for memory bandwidth considerations. It updates the VM creation, > rebind, and page table update paths to check this flag and fall back to the > primary tile's root page table when mirroring is disabled. > > Link: https://patch.msgid.link/20260228013501.106680-18-matthew.brost@intel.com > >> diff --git a/drivers/gpu/drm/xe/xe_vm.c b/drivers/gpu/drm/xe/xe_vm.c >> --- a/drivers/gpu/drm/xe/xe_vm.c >> +++ b/drivers/gpu/drm/xe/xe_vm.c > > [ ... ] > >> @@ -1738,7 +1748,8 @@ struct xe_vm *xe_vm_create(struct xe_device *xe, u32 flags, struct xe_file *xef) >> >> for_each_tile(tile, xe, id) { >> if (flags & XE_VM_FLAG_MIGRATION && >> - tile->id != XE_VM_FLAG_TILE_ID(flags)) >> + tile->id != XE_VM_FLAG_TILE_ID(flags) && >> + (vm->xe->info.has_pt_mirror || id)) >> continue; >> >> vm->pt_root[id] = xe_pt_create(vm, tile, xe->info.vm_max_level, > > [Severity: Medium] > Does this code unnecessarily allocate page table roots and scratch structures > for secondary tiles when PT mirroring is disabled? > > For standard user VMs, XE_VM_FLAG_MIGRATION is unset, meaning this if > condition evaluates to false and we proceed to allocate pt_root[id] for all > tiles. > > However, when vm->xe->info.has_pt_mirror is false, xe_vm_pt_root() > unconditionally returns pt_root[0]. This means the allocated pt_root[id] > (and associated scratch page tables up to vm_max_level) for id > 0 will never > be used, permanently wasting VRAM for the lifetime of every VM. > > Should there be an explicit check to skip secondary tile allocations when > has_pt_mirror is false, regardless of whether it is a migration VM? >