Re: [PATCH v6 01/12] drm/xe/uapi: Add UAPI support for purgeable buffer objects

Thomas Hellström
2026-03-10T08:31:32Z

On Tue, 2026-03-03 at 20:49 +0530, Arvind Yadav wrote:
> 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 page faults return SIGBUS
>   - GPU page faults fail with -EACCES
>
>   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:
>   - Add PURGED state for read-only status, change ioctl to DRM_IOWR,
>     add retained field for i915 compatibility
>
> v3:
>   - UAPI rule should not be changed (Matthew Brost)
>   - Make 'retained' a userptr (Matthew Brost)
>
> v4:
>   - You cannot make this part of the union (purge_state_val) larger
>     than the existing union (16 bytes). So just drop the '__u64
> reserved'
>     field. (Matt)
>
> v5:
>   - Update UAPI documentation to clarify retained must be initialized
>     to 0(Thomas)
>
> v6:
>   - 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)
>   - Rename retained to retained_ptr. (Jose)
>
> Cc: Jose Souze <jose.souza@intel.com>
> Cc: Matthew Brost <matthew.brost@intel.com>
> Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> Signed-off-by: Himal Prasad Ghimiray
> <himal.prasad.ghimiray@intel.com>
> Signed-off-by: Arvind Yadav <arvind.yadav@intel.com>

Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>

> ---
>  include/uapi/drm/xe_drm.h | 60
> +++++++++++++++++++++++++++++++++++++++
>  1 file changed, 60 insertions(+)
>
> diff --git a/include/uapi/drm/xe_drm.h b/include/uapi/drm/xe_drm.h
> index ef2565048bdf..42aedd30189d 100644
> --- a/include/uapi/drm/xe_drm.h
> +++ b/include/uapi/drm/xe_drm.h
> @@ -425,6 +425,7 @@ 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
> << 4)
>  #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 +2192,63 @@ 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.
> + *
> + * Supported values for @purge_state_val.val:
> + *  - DRM_XE_VMA_PURGEABLE_STATE_WILLNEED (0): Marks
> BO as needed.
> + *    If BO was previously purged, returns
> retained=0 (backing store lost).
> + *
> + *  - 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. Returns
> retained=1 if not purged yet.
> + *
> + *    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.
> + *
> + *    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 */