Re: [PATCH v6 17/24] drm/xe: Add device flag to enable PT mirroring across tiles

[sashiko-bot@kernel.org]

Thank you for your contribution! 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.
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commit 34b539e899a7643dc15dedad6a3cd229e0c0f4d5
Author: Matthew Brost <matthew.brost@intel.com>

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?

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Sashiko AI review · https://sashiko.dev/#/patchset/20260904211613.3934307-1-matthew.brost@intel.com?part=17