[RFC v2 3/9] drm/xe/bo: Prevent purging of shared buffer objects

[Arvind Yadav <arvind.yadav@intel.com>]

Introduce the `xe_bo_is_shared_locked()` inline helper to determine if a
buffer object is shared across multiple clients or drivers. A buffer is
considered shared if it is exported via dma-buf, imported, or has a
handle count greater than one.

This check is critical for safely implementing purgeable memory. Purging
a buffer that is shared would lead to data corruption for other clients
that still hold a reference to it.

The kernel cannot safely determine when all clients are done with a
shared buffer, so shared BOs must never be marked DONTNEED or purged.

The new helper is used in two key locations:
1.  In `xe_vm_madvise_purgeable_bo()`, to prevent userspace from
    successfully marking a shared buffer as `DONTNEED`. This is the
    primary safeguard against incorrect usage.

2.  In `xe_bo_move()`, as a final safety check before the kernel
    initiates a purge during eviction. This ensures that even if a
    shared buffer were somehow marked `DONTNEED`, it would not be
    purged.

Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Signed-off-by: Arvind Yadav <arvind.yadav@intel.com>
---
 drivers/gpu/drm/xe/xe_bo.h | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/drivers/gpu/drm/xe/xe_bo.h b/drivers/gpu/drm/xe/xe_bo.h
index b0a31c77e612..97edb38bf1ed 100644
--- a/drivers/gpu/drm/xe/xe_bo.h
+++ b/drivers/gpu/drm/xe/xe_bo.h
@@ -478,4 +478,34 @@ static inline bool xe_bo_is_mem_type(struct xe_bo *bo, u32 mem_type)
 	xe_bo_assert_held(bo);
 	return bo->ttm.resource->mem_type == mem_type;
 }
+
+/**
+ * xe_bo_is_shared_locked - Check if a buffer object is shared
+ * @bo: The buffer object to check
+ *
+ * Determines if a buffer object is considered shared, which includes:
+ * - Exported via dma-buf (obj->dma_buf is set)
+ * - Imported from another driver (obj->import_attach is set)
+ * - Referenced by multiple clients (handle_count > 1)
+ *
+ * This check is used to prevent data loss on shared content by avoiding
+ * certain operations like purging on buffers that other processes or
+ * drivers might still be using.
+ *
+ * Return: true if the buffer object is shared, false otherwise.
+ */
+static inline bool xe_bo_is_shared_locked(const struct xe_bo *bo)
+{
+	const struct drm_gem_object *obj = &bo->ttm.base;
+
+	dma_resv_assert_held(obj->resv);
+
+	if (obj->dma_buf || obj->import_attach)
+		return true;
+
+	if (obj->handle_count > 1)
+		return true;
+
+	return false;
+}
 #endif
-- 
2.43.0