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

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

On 10-12-2025 11:03, Matthew Brost wrote:
> 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.
Noted.
>
> 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.


Noted. Please update us once you have clarity on whether changing from 
DRM_IOW to DRM_IOWR is allowed, for future reference.

~Arvind

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