[PATCH i-g-t v6 1/9] drm-uapi/xe_drm: Add UAPI support for purgeable buffer objects

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

From: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>

Extend the DRM_XE_MADVISE ioctl to support purgeable buffer object
management by adding DRM_XE_VMA_ATTR_PURGEABLE_STATE attribute type.

This allows userspace applications to provide memory usage hints to
the kernel for better memory management under pressure:

- WILLNEED: Buffer is needed and should not be purged. If the BO was
  previously purged, retained field returns 0 indicating backing store
  was lost (once purged, always purged semantics matching i915).

- DONTNEED: Buffer is not currently needed and may be purged by the
  kernel under memory pressure to free resources. Only applies to
  non-shared BOs.

  To prevent undefined behavior, the following operations are blocked
  while a BO is in DONTNEED state:
  - New mmap() operations return -EBUSY
  - VM_BIND operations return -EBUSY
  - New dma-buf exports return -EBUSY
  - CPU/GPU page faults return SIGBUS

  This ensures applications cannot use a BO while marked as DONTNEED,
  preventing erratic behavior when the kernel purges the backing store.

The implementation includes a 'retained' output field (matching i915's
drm_i915_gem_madvise.retained) that indicates whether the BO's backing
store still exists (1) or has been purged (0).

Added DRM_XE_QUERY_CONFIG_FLAG_HAS_PURGING_SUPPORT flag to allow
userspace to detect kernel support for purgeable buffer objects
before attempting to use the feature.

v2:
  - Update UAPI documentation to clarify retained must be initialized
    to 0(Thomas)

v5:
  - Document DONTNEED BO access blocking behavior to prevent undefined
    behavior and clarify uAPI contract (Thomas, Matt)
  - Add query flag DRM_XE_QUERY_CONFIG_FLAG_HAS_PURGING_SUPPORT for
    feature detection. (Jose)
  - Renamed retained to retained_ptr. (Jose)

v6:
  - Updated UAPI documentation as suggested. (Jose)

Cc: Nishit Sharma <nishit.sharma@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Pravalika Gurram <pravalika.gurram@intel.com>
Signed-off-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Signed-off-by: Arvind Yadav <arvind.yadav@intel.com>
---
 include/drm-uapi/xe_drm.h | 69 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 69 insertions(+)

diff --git a/include/drm-uapi/xe_drm.h b/include/drm-uapi/xe_drm.h
index 077e66a68..cf0eb3f99 100644
--- a/include/drm-uapi/xe_drm.h
+++ b/include/drm-uapi/xe_drm.h
@@ -431,6 +431,7 @@ struct drm_xe_query_config {
 	#define DRM_XE_QUERY_CONFIG_FLAG_HAS_LOW_LATENCY	(1 << 1)
 	#define DRM_XE_QUERY_CONFIG_FLAG_HAS_CPU_ADDR_MIRROR	(1 << 2)
 	#define DRM_XE_QUERY_CONFIG_FLAG_HAS_NO_COMPRESSION_HINT (1 << 3)
+	#define DRM_XE_QUERY_CONFIG_FLAG_HAS_PURGING_SUPPORT	(1 << 5)
 #define DRM_XE_QUERY_CONFIG_MIN_ALIGNMENT		2
 #define DRM_XE_QUERY_CONFIG_VA_BITS			3
 #define DRM_XE_QUERY_CONFIG_MAX_EXEC_QUEUE_PRIORITY	4
@@ -2067,6 +2068,7 @@ struct drm_xe_query_eu_stall {
  *  - DRM_XE_MEM_RANGE_ATTR_PREFERRED_LOC: Set preferred memory location.
  *  - DRM_XE_MEM_RANGE_ATTR_ATOMIC: Set atomic access policy.
  *  - DRM_XE_MEM_RANGE_ATTR_PAT: Set page attribute table index.
+ *  - DRM_XE_VMA_ATTR_PURGEABLE_STATE: Set purgeable state for BOs.
  *
  * Example:
  *
@@ -2099,6 +2101,7 @@ struct drm_xe_madvise {
 #define DRM_XE_MEM_RANGE_ATTR_PREFERRED_LOC	0
 #define DRM_XE_MEM_RANGE_ATTR_ATOMIC		1
 #define DRM_XE_MEM_RANGE_ATTR_PAT		2
+#define DRM_XE_VMA_ATTR_PURGEABLE_STATE		3
 	/** @type: type of attribute */
 	__u32 type;
 
@@ -2189,6 +2192,72 @@ struct drm_xe_madvise {
 			/** @pat_index.reserved: Reserved */
 			__u64 reserved;
 		} pat_index;
+
+		/**
+		 * @purge_state_val: Purgeable state configuration
+		 *
+		 * Used when @type == DRM_XE_VMA_ATTR_PURGEABLE_STATE.
+		 *
+		 * Configures the purgeable state of buffer objects in the specified
+		 * virtual address range. This allows applications to hint to the kernel
+		 * about bo's usage patterns for better memory management.
+		 *
+		 * By default all VMAs are in WILLNEED state.
+		 *
+		 * Supported values for @purge_state_val.val:
+		 *  - DRM_XE_VMA_PURGEABLE_STATE_WILLNEED (0): Marks BO as needed.
+		 *    If the BO was previously purged, the kernel sets the __u32 at
+		 *    @retained_ptr to 0 (backing store lost) so the application knows
+		 *    it must recreate the BO.
+		 *
+		 *  - DRM_XE_VMA_PURGEABLE_STATE_DONTNEED (1): Marks BO as not currently
+		 *    needed. Kernel may purge it under memory pressure to reclaim memory.
+		 *    Only applies to non-shared BOs. The kernel sets the __u32 at
+		 *    @retained_ptr to 1 if the backing store still exists (not yet purged),
+		 *    or 0 if it was already purged.
+		 *
+		 *    Important: Once marked as DONTNEED, touching the BO's memory
+		 *    is undefined behavior. It may succeed temporarily (before the
+		 *    kernel purges the backing store) but will suddenly fail once
+		 *    the BO transitions to PURGED state.
+		 *
+		 *    To transition back: use WILLNEED and check @retained_ptr —
+		 *    if 0, backing store was lost and the BO must be recreated.
+		 *
+		 *    The following operations are blocked in DONTNEED state to
+		 *    prevent the BO from being re-mapped after madvise:
+		 *    - New mmap() calls: Fail with -EBUSY
+		 *    - VM_BIND operations: Fail with -EBUSY
+		 *    - New dma-buf exports: Fail with -EBUSY
+		 *    - CPU page faults (existing mmap): Fail with SIGBUS
+		 *    - GPU page faults (fault-mode VMs): Fail with -EACCES
+		 */
+		struct {
+#define DRM_XE_VMA_PURGEABLE_STATE_WILLNEED	0
+#define DRM_XE_VMA_PURGEABLE_STATE_DONTNEED	1
+			/** @purge_state_val.val: value for DRM_XE_VMA_ATTR_PURGEABLE_STATE */
+			__u32 val;
+
+			/** @purge_state_val.pad: MBZ */
+			__u32 pad;
+			/**
+			 * @purge_state_val.retained_ptr: Pointer to a __u32 output
+			 * field for backing store status.
+			 *
+			 * Userspace must initialize the __u32 value at this address
+			 * to 0 before the ioctl. Kernel writes a __u32 after the
+			 * operation:
+			 * - 1 if backing store exists (not purged)
+			 * - 0 if backing store was purged
+			 *
+			 * If userspace fails to initialize to 0, ioctl returns -EINVAL.
+			 * This ensures a safe default (0 = assume purged) if kernel
+			 * cannot write the result.
+			 *
+			 * Similar to i915's drm_i915_gem_madvise.retained field.
+			 */
+			__u64 retained_ptr;
+		} purge_state_val;
 	};
 
 	/** @reserved: Reserved */
-- 
2.43.0