Re: [PATCH 1/2] drm/xe/dma-buf: keep non-p2p imported buffers in system memory

[Matthew Brost <matthew.brost@intel.com>]

On Mon, Sep 28, 2026 at 05:48:22PM +0100, Matthew Auld wrote:
> When an exported buffer is mapped by a foreign device lacking peer-to-peer
> DMA support (such as an integrated GPU for display offload),
> xe_dma_buf_map() migrates the buffer to XE_PL_TT so that system memory
> pages can be accessed by the importer.
> 
> However, since commit 5c87fee3c96c ("drm/xe: Attempt to bring bos back to
> VRAM after eviction"), XE_PL_TT is marked with TTM_PL_FLAG_FALLBACK in
> the buffer's placement. When DMABUF_MOVE_NOTIFY was enabled by default
> (meaning dynamic attachments are no longer pinned on map), the next
> xe_bo_validate() during render submission sees TT as a fallback placement
> and attempts to migrate the buffer back to VRAM.
> 
> On the next frame, the foreign importer accesses the buffer, requiring
> another migration to TT, resulting in a continuous ping-pong between
> VRAM and system memory every frame and causing severe rendering performance
> degradations.
> 
> To fix this, introduce xe_bo_migrate_tt_sticky() which records a
> temporary sticky placement in XE_PL_TT on successful migration. Subsequent
> validations use this sticky placement to keep the buffer in system memory
> until the non-p2p attachment is detached or memory pressure forces a
> fallback to the default placement.
> 
> User is reporting what looks to be exactly this, with horrible
> performance when DMABUF_MOVE_NOTIFY was enabled by default.
> 

The patch looks functionally correct but couldn't we just clear
TTM_PL_FLAG_FALLBACK in xe_bo_migrate for certain callers? Then restore
it in other places?

Matt

> Assisted-by: LLM
> Link: https://gitlab.freedesktop.org/drm/xe/kernel/-/work_items/9415
> Fixes: 5c87fee3c96c ("drm/xe: Attempt to bring bos back to VRAM after eviction")
> Signed-off-by: Matthew Auld <matthew.auld@intel.com>
> Cc: <stable@vger.kernel.org> # v6.12+
> Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> Cc: Matthew Brost <matthew.brost@intel.com>
> ---
>  drivers/gpu/drm/xe/xe_bo.c       | 59 ++++++++++++++++++++++++++++++--
>  drivers/gpu/drm/xe/xe_bo.h       |  3 ++
>  drivers/gpu/drm/xe/xe_bo_types.h |  4 +++
>  drivers/gpu/drm/xe/xe_dma_buf.c  | 16 ++++++++-
>  4 files changed, 79 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/xe/xe_bo.c b/drivers/gpu/drm/xe/xe_bo.c
> index 6921b6967330..1c1eab0a7434 100644
> --- a/drivers/gpu/drm/xe/xe_bo.c
> +++ b/drivers/gpu/drm/xe/xe_bo.c
> @@ -3324,6 +3324,9 @@ int xe_bo_validate(struct xe_bo *bo, struct xe_vm *vm, bool allow_res_evict,
>  		.no_wait_gpu = false,
>  		.gfp_retry_mayfail = true,
>  	};
> +	struct ttm_placement *placement = bo->sticky_placement.num_placement ?
> +					  &bo->sticky_placement :
> +					  &bo->placement;
>  	int ret;
>  
>  	if (xe_bo_is_pinned(bo))
> @@ -3340,7 +3343,12 @@ int xe_bo_validate(struct xe_bo *bo, struct xe_vm *vm, bool allow_res_evict,
>  	xe_vm_set_validating(vm, allow_res_evict);
>  	trace_xe_bo_validate(bo);
>  	xe_validation_assert_exec(xe_bo_device(bo), exec, &bo->ttm.base);
> -	ret = ttm_bo_validate(&bo->ttm, &bo->placement, &ctx);
> +	ret = ttm_bo_validate(&bo->ttm, placement, &ctx);
> +	if (ret && ret != -EINTR && ret != -ERESTARTSYS &&
> +	    bo->sticky_placement.num_placement) {
> +		xe_bo_reset_sticky_placement(bo);
> +		ret = ttm_bo_validate(&bo->ttm, &bo->placement, &ctx);
> +	}
>  	xe_vm_clear_validating(vm, allow_res_evict);
>  
>  	return ret;
> @@ -3891,6 +3899,7 @@ int xe_bo_migrate(struct xe_bo *bo, u32 mem_type, struct ttm_operation_ctx *tctx
>  	};
>  	struct ttm_placement placement;
>  	struct ttm_place requested;
> +	int ret;
>  
>  	xe_bo_assert_held(bo);
>  	tctx = tctx ? tctx : &ctx;
> @@ -3922,7 +3931,53 @@ int xe_bo_migrate(struct xe_bo *bo, u32 mem_type, struct ttm_operation_ctx *tctx
>  
>  	if (!tctx->no_wait_gpu)
>  		xe_validation_assert_exec(xe_bo_device(bo), exec, &bo->ttm.base);
> -	return ttm_bo_validate(&bo->ttm, &placement, tctx);
> +	ret = ttm_bo_validate(&bo->ttm, &placement, tctx);
> +	if (!ret)
> +		xe_bo_reset_sticky_placement(bo);
> +	return ret;
> +}
> +
> +/**
> + * xe_bo_migrate_tt_sticky - Migrate an object to TT and record its placement as sticky
> + * @bo: The buffer object to migrate.
> + * @tctx: The ttm_operation_ctx to use for migration, or NULL for default.
> + * @exec: The drm_exec transaction to use for exhaustive eviction.
> + *
> + * Like xe_bo_migrate() to XE_PL_TT, but on success records the resulting placement
> + * so that subsequent validations try to keep the object in TT instead of falling
> + * back to the default placement. The stickiness is removed if validation falls
> + * back to the default placement (e.g. under memory pressure), on any non-sticky
> + * migration, or via an explicit call to xe_bo_reset_sticky_placement().
> + *
> + * Return: 0 on success. Negative error code on failure.
> + */
> +int xe_bo_migrate_tt_sticky(struct xe_bo *bo,
> +			    struct ttm_operation_ctx *tctx,
> +			    struct drm_exec *exec)
> +{
> +	int ret;
> +
> +	ret = xe_bo_migrate(bo, XE_PL_TT, tctx, exec);
> +	if (!ret) {
> +		xe_place_from_ttm_type(XE_PL_TT, &bo->sticky_place);
> +		bo->sticky_placement = (struct ttm_placement){
> +			.num_placement = 1,
> +			.placement = &bo->sticky_place,
> +		};
> +	}
> +	return ret;
> +}
> +
> +/**
> + * xe_bo_reset_sticky_placement - Reset sticky placement for an object
> + * @bo: The buffer object whose sticky placement should be cleared.
> + *
> + * Clear any sticky placement recorded by xe_bo_migrate_tt_sticky(),
> + * returning subsequent validations to the default placement.
> + */
> +void xe_bo_reset_sticky_placement(struct xe_bo *bo)
> +{
> +	bo->sticky_placement.num_placement = 0;
>  }
>  
>  /**
> diff --git a/drivers/gpu/drm/xe/xe_bo.h b/drivers/gpu/drm/xe/xe_bo.h
> index 861b1be231de..7327628070f2 100644
> --- a/drivers/gpu/drm/xe/xe_bo.h
> +++ b/drivers/gpu/drm/xe/xe_bo.h
> @@ -434,6 +434,9 @@ bool xe_bo_can_migrate(struct xe_bo *bo, u32 mem_type);
>  
>  int xe_bo_migrate(struct xe_bo *bo, u32 mem_type, struct ttm_operation_ctx *ctc,
>  		  struct drm_exec *exec);
> +int xe_bo_migrate_tt_sticky(struct xe_bo *bo, struct ttm_operation_ctx *ctc,
> +			    struct drm_exec *exec);
> +void xe_bo_reset_sticky_placement(struct xe_bo *bo);
>  int xe_bo_evict(struct xe_bo *bo, struct drm_exec *exec);
>  
>  int xe_bo_evict_pinned(struct xe_bo *bo);
> diff --git a/drivers/gpu/drm/xe/xe_bo_types.h b/drivers/gpu/drm/xe/xe_bo_types.h
> index 8ec4a01a0092..253a1dba55a7 100644
> --- a/drivers/gpu/drm/xe/xe_bo_types.h
> +++ b/drivers/gpu/drm/xe/xe_bo_types.h
> @@ -56,6 +56,10 @@ struct xe_bo {
>  	struct ttm_place placements[XE_BO_MAX_PLACEMENTS];
>  	/** @placement: current placement for this BO */
>  	struct ttm_placement placement;
> +	/** @sticky_placement: target placement from forced migration */
> +	struct ttm_placement sticky_placement;
> +	/** @sticky_place: place for sticky_placement */
> +	struct ttm_place sticky_place;
>  	/** @ggtt_node: Array of GGTT nodes if this BO is mapped in the GGTTs */
>  	struct xe_ggtt_node *ggtt_node[XE_MAX_TILES_PER_DEVICE];
>  	/** @vmap: iosys map of this buffer */
> diff --git a/drivers/gpu/drm/xe/xe_dma_buf.c b/drivers/gpu/drm/xe/xe_dma_buf.c
> index 6a85b292dee7..b973579a69b8 100644
> --- a/drivers/gpu/drm/xe/xe_dma_buf.c
> +++ b/drivers/gpu/drm/xe/xe_dma_buf.c
> @@ -59,6 +59,20 @@ static void xe_dma_buf_detach(struct dma_buf *dmabuf,
>  			      struct dma_buf_attachment *attach)
>  {
>  	struct drm_gem_object *obj = attach->dmabuf->priv;
> +	struct xe_bo *bo = gem_to_xe_bo(obj);
> +	bool has_non_p2p = false;
> +	struct dma_buf_attachment *a;
> +
> +	dma_resv_lock(dmabuf->resv, NULL);
> +	list_for_each_entry(a, &dmabuf->attachments, node) {
> +		if (!a->peer2peer) {
> +			has_non_p2p = true;
> +			break;
> +		}
> +	}
> +	if (!has_non_p2p)
> +		xe_bo_reset_sticky_placement(bo);
> +	dma_resv_unlock(dmabuf->resv);
>  
>  	xe_pm_runtime_put(to_xe_device(obj->dev));
>  }
> @@ -129,7 +143,7 @@ static struct sg_table *xe_dma_buf_map(struct dma_buf_attachment *attach,
>  
>  	if (!xe_bo_is_pinned(bo)) {
>  		if (!attach->peer2peer)
> -			r = xe_bo_migrate(bo, XE_PL_TT, NULL, exec);
> +			r = xe_bo_migrate_tt_sticky(bo, NULL, exec);
>  		else
>  			r = xe_bo_validate(bo, NULL, false, exec);
>  		if (r)
> -- 
> 2.55.0
>