[RFC v2 7/9] drm/xe/vm: Prevent binding of purged buffer objects

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

Add validation in xe_vm_bind_ioctl_validate_bo() to reject MAP and
PREFETCH operations on purged buffer objects with -EINVAL.

Problem:
When a BO is purged (XE_MADV_PURGEABLE_PURGED state), its backing pages
have been freed by the kernel. Without this check, VM_BIND operations
would proceed:

 1. DRM_XE_VM_BIND_OP_MAP: Attempts to create GPU mappings to freed memory
    - xe_vma_ops_alloc() creates VMA pointing to invalid BO
    - Page tables populated with stale/invalid addresses
    - GPU access leads to undefined behavior or hangs

 2. DRM_XE_VM_BIND_OP_PREFETCH: Attempts to migrate non-existent pages
    - Triggers BO validation in TTM
    - ttm_bo_validate() fails or crashes (no backing store)
    - Wasted work for permanently invalid BO

With this check:
  - MAP/PREFETCH immediately fail with -EINVAL at ioctl boundary
  - Clear error message at syscall (better UX than deferred GPU hang)
  - Prevents creation of invalid GPU page table entries

v2:
  - Clarify that purged BOs are permanently invalid (i915 semantics)
  - Remove incorrect claim about madvise(WILLNEED) restoring purged BOs

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_vm.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/gpu/drm/xe/xe_vm.c b/drivers/gpu/drm/xe/xe_vm.c
index d03e69524369..cc946bff9607 100644
--- a/drivers/gpu/drm/xe/xe_vm.c
+++ b/drivers/gpu/drm/xe/xe_vm.c
@@ -3482,6 +3482,13 @@ static int xe_vm_bind_ioctl_validate_bo(struct xe_device *xe, struct xe_bo *bo,
 		return -EINVAL;
 	}
 
+	/* Purged BOs are permanently invalid; reject new MAP/PREFETCH. */
+	if (XE_IOCTL_DBG(xe,
+			 xe_bo_is_purged(bo) &&
+			 (op == DRM_XE_VM_BIND_OP_MAP ||
+			  op == DRM_XE_VM_BIND_OP_PREFETCH)))
+		return -EINVAL;
+
 	/*
 	 * Some platforms require 64k VM_BIND alignment,
 	 * specifically those with XE_VRAM_FLAGS_NEED64K.
-- 
2.43.0