[PATCH] drm/xe/dma-buf: Validate BOs on export to charge the exporter

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

xe_gem_prime_export() calls ttm_bo_setup_export() to make sure the
exporter is charged for the memory backing an exported BO. However,
that only populates a TTM_PL_SYSTEM tt. A BO that has been evicted, or
that has deferred backing, will instead get its memory allocated when
the importer maps it. For p2p imports this happens through
xe_bo_validate() in xe_dma_buf_map(), so the importer, not the
exporter, is charged for the allocation.

Call xe_bo_validate() at export time instead, so that the BO is moved
into its preferred placement and the exporter is charged.

Do the purgeable checks and the validation in the same
xe_validation_guard() transaction, to avoid locking the BO twice.

The commit message and code was drafted using LLM.

Link: https://patchwork.freedesktop.org/patch/747480/?series=172524&rev=1
Fixes: 91494dee1091 ("xe: populate buffers before exporting them.")
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: intel-xe@lists.freedesktop.org
Cc: <stable@vger.kernel.org> # v6.18+
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
---
 drivers/gpu/drm/xe/xe_dma_buf.c | 76 +++++++++++++++++----------------
 1 file changed, 39 insertions(+), 37 deletions(-)

diff --git a/drivers/gpu/drm/xe/xe_dma_buf.c b/drivers/gpu/drm/xe/xe_dma_buf.c
index 5bf13dc4b61d..90c9b90de552 100644
--- a/drivers/gpu/drm/xe/xe_dma_buf.c
+++ b/drivers/gpu/drm/xe/xe_dma_buf.c
@@ -263,60 +263,62 @@ struct drm_gem_object *xe_dma_buf_to_gem(struct drm_device *dev,
 struct dma_buf *xe_gem_prime_export(struct drm_gem_object *obj, int flags)
 {
 	struct xe_bo *bo = gem_to_xe_bo(obj);
+	struct xe_validation_ctx ctx;
+	struct drm_exec exec;
 	struct dma_buf *buf;
-	struct ttm_operation_ctx ctx = {
-		.interruptible = true,
-		.no_wait_gpu = true,
-		/* We opt to avoid OOM on system pages allocations */
-		.gfp_retry_mayfail = true,
-		.allow_res_evict = false,
-	};
-	int ret;
+	int ret = 0;
 
 	if (bo->vm)
 		return ERR_PTR(-EPERM);
 
-	/*
-	 * Reject exporting purgeable BOs. DONTNEED BOs can be purged
-	 * at any time, making the exported dma-buf unusable. Purged BOs
-	 * have no backing store and are permanently invalid.
-	 */
-	ret = xe_bo_lock(bo, true);
-	if (ret)
-		return ERR_PTR(ret);
+	xe_validation_guard(&ctx, &xe_bo_device(bo)->val, &exec,
+			    (struct xe_val_flags) {.interruptible = true}, ret) {
+		ret = drm_exec_lock_obj(&exec, &bo->ttm.base);
+		drm_exec_retry_on_contention(&exec);
+		if (ret)
+			break;
 
-	if (xe_bo_madv_is_dontneed(bo)) {
-		ret = -EBUSY;
-		goto out_unlock;
-	}
+		/*
+		 * Reject exporting purgeable BOs. DONTNEED BOs can be purged
+		 * at any time, making the exported dma-buf unusable. Purged BOs
+		 * have no backing store and are permanently invalid.
+		 */
+		if (xe_bo_madv_is_dontneed(bo)) {
+			ret = -EBUSY;
+			break;
+		}
 
-	if (xe_bo_is_purged(bo)) {
-		ret = -EINVAL;
-		goto out_unlock;
-	}
+		if (xe_bo_is_purged(bo)) {
+			ret = -EINVAL;
+			break;
+		}
 
-	xe_bo_willneed_get_locked(bo);
-	xe_bo_unlock(bo);
+		/*
+		 * Make sure the BO is backed in its preferred placement so that
+		 * the exporter, rather than the importer, is charged for the
+		 * memory allocation.
+		 */
+		ret = xe_bo_validate(bo, NULL, false, &exec);
+		drm_exec_retry_on_contention(&exec);
+		xe_validation_retry_on_oom(&ctx, &ret);
+		if (ret)
+			break;
 
-	ret = ttm_bo_setup_export(&bo->ttm, &ctx);
+		xe_bo_willneed_get_locked(bo);
+	}
 	if (ret)
-		goto out_put;
+		return ERR_PTR(ret);
 
 	buf = drm_gem_prime_export(obj, flags);
 	if (IS_ERR(buf)) {
-		ret = PTR_ERR(buf);
-		goto out_put;
+		xe_bo_lock(bo, false);
+		xe_bo_willneed_put_locked(bo);
+		xe_bo_unlock(bo);
+		return buf;
 	}
 
 	buf->ops = &xe_dmabuf_ops;
 	return buf;
-
-out_put:
-	xe_bo_lock(bo, false);
-	xe_bo_willneed_put_locked(bo);
-out_unlock:
-	xe_bo_unlock(bo);
-	return ERR_PTR(ret);
 }
 
 static struct drm_gem_object *
-- 
2.56.0