Re: [RFC v3 1/8] drm/xe/uapi: Add UAPI support for purgeable buffer objects

[Matthew Brost <matthew.brost@intel.com>]

On Wed, Dec 10, 2025 at 10:00:45AM +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:
> 
> 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.
> 
> 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).
> 
> 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)
> 
> 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>
> ---
>  include/uapi/drm/xe_drm.h | 39 +++++++++++++++++++++++++++++++++++++++
>  1 file changed, 39 insertions(+)
> 
> diff --git a/include/uapi/drm/xe_drm.h b/include/uapi/drm/xe_drm.h
> index 876a076fa6c0..fc0dada62dd0 100644
> --- a/include/uapi/drm/xe_drm.h
> +++ b/include/uapi/drm/xe_drm.h
> @@ -2079,6 +2079,7 @@ 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;
>  
> @@ -2157,6 +2158,44 @@ 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 purged, returns retained=0 (backing store lost).
> +		 *
> +		 *  - DRM_XE_VMA_PURGEABLE_STATE_DONTNEED (1): Hints that BO is not
> +		 *    currently needed. Kernel may purge it under memory pressure.
> +		 *    Only applies to non-shared BOs. Returns retained=1 if not purged.
> +		 */
> +		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 */
> +			__u32 pad;
> +			/**
> +			 * @purge_state_val.retained: Pointer to output field for backing
> +			 * store status.
> +			 *
> +			 * Userspace provides a pointer to u32. Kernel writes to it:
> +			 * 1 if backing store exists, 0 if purged.
> +			 * Similar to i915's drm_i915_gem_madvise.retained field.
> +			 */
> +			__u64 retained;
> +			/** @purge_state_val.reserved: Reserved */
> +			__u64 reserved;

You cannot make this part of the union (purge_state_val) larger than the
existing union (16 bytes) or the bit layout of the IOCTL changes and the
uAPI breaks.

So just drop the '__u64 reserved' field.

I'll also follow up if we can change an IOCTL from DRM_IOW to DRM_IOWR
too. I honestly don't know if that is allowed or not.

Matt

> +		} purge_state_val;
>  	};
>  
>  	/** @reserved: Reserved */
> -- 
> 2.43.0
>