Re: [PATCH] drm/xe/bo: optimise TT population for DONTNEED BOs

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

On Thu, 2026-07-23 at 14:34 +0100, Matthew Auld wrote:
> When a VRAM buffer object is marked as DONTNEED, like in Mesa, the
> driver skips migrating its contents to system memory and instead
> purges
> the backing store during eviction (via xe_ttm_bo_purge).
> 
> However, xe_evict_flags() still returns tt_placement (XE_PL_TT) for
> VRAM
> BOs even if they were marked DONTNEED.  This causes
> ttm_bo_handle_move_mem() to call always call ttm_bo_populate() to
> allocate destination system pages, only for those pages to be
> immediately freed right after when xe_bo_move() calls
> xe_ttm_bo_purge().
> 
> Fix this by changing xe_evict_flags() to return sys_placement
> (XE_PL_SYSTEM) for DONTNEED BOs.  This causes TTM to skip the
> population
> step while still ensuring that TTM calls xe_bo_move(), so
> xe_ttm_bo_purge() still triggers.
> 
> Assisted-by: Copilot:gemini-3.1-pro-preview
> Signed-off-by: Matthew Auld <matthew.auld@intel.com>
> Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> Cc: Matthew Brost <matthew.brost@intel.com>
> ---
>  drivers/gpu/drm/xe/xe_bo.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/gpu/drm/xe/xe_bo.c b/drivers/gpu/drm/xe/xe_bo.c
> index c266fa6bade1..1daa80079608 100644
> --- a/drivers/gpu/drm/xe/xe_bo.c
> +++ b/drivers/gpu/drm/xe/xe_bo.c
> @@ -341,6 +341,11 @@ static void xe_evict_flags(struct
> ttm_buffer_object *tbo,
>  		return;
>  	}
>  
> +	if (xe_bo_madv_is_dontneed(bo)) {
> +		*placement = sys_placement;
> +		return;
> +	}
> +

It might be worth adding a comment here, since it's tempting to add a
completely empty placement, like I think amdgpu does. But IIRC then the
move callback won't even get called and hence no move_notify which
triggers the deferred rebind.

/Thomas



>  	/*
>  	 * For xe, sg bos that are evicted to system just triggers a
>  	 * rebind of the sg list upon subsequent validation to
> XE_PL_TT.