Re: [PATCH 2/2] drm/xe/vm: make PREFETCH to TT sticky

[Matthew Auld <matthew.auld@intel.com>]

On 02/10/2026 13:43, Thomas Hellström wrote:
> On Mon, 2026-09-28 at 17:48 +0100, Matthew Auld wrote:
>> When userspace explicitly requests DRM_XE_VM_BIND_OP_PREFETCH to
>> system
>> memory (XE_PL_TT), xe_bo_migrate() moves the buffer object to TT.
>> However, because XE_PL_TT is marked with TTM_PL_FLAG_FALLBACK in the
>> buffer's default placement (introduced in commit 5c87fee3c96c
>> ("drm/xe:
>> Attempt to bring bos back to VRAM after eviction")), any subsequent
>> validation during exec immediately migrates the buffer back to VRAM,
>> completely undoing the userspace prefetch request.
>>
>> Use xe_bo_migrate_tt_sticky() when prefetching to XE_PL_TT so that
>> the
>> buffer remains resident in system memory across subsequent
>> validations.
>>
>> Assisted-by: LLM
>> Reported-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
>> Fixes: 5c87fee3c96c ("drm/xe: Attempt to bring bos back to VRAM after
>> eviction")
>> Signed-off-by: Matthew Auld <matthew.auld@intel.com>
>> Cc: Matthew Brost <matthew.brost@intel.com>
>> Cc: <stable@vger.kernel.org> # v6.12+
> 
> I think we need somewhere to clear this stickiness. Perhaps if
> xe_bo_migrate() is called with another destination than the sticky one,
> we'd clear it?

In the other patch in bo_migrate() there is:

+	ret = ttm_bo_validate(&bo->ttm, &placement, tctx);
+	if (!ret)
+		xe_bo_reset_sticky_placement(bo);
+	return ret;
+}

So if something successfully uses a non-sticky migrate(), it will reset. 
Is this enough?

> 
> Thanks,
> Thomas
> 
> 
> 
> 
>> ---
>>   drivers/gpu/drm/xe/xe_vm.c | 5 ++++-
>>   1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/xe/xe_vm.c b/drivers/gpu/drm/xe/xe_vm.c
>> index 425c678f4480..2068e0cf50ff 100644
>> --- a/drivers/gpu/drm/xe/xe_vm.c
>> +++ b/drivers/gpu/drm/xe/xe_vm.c
>> @@ -3406,7 +3406,10 @@ static int op_lock_and_prep(struct drm_exec
>> *exec, struct xe_vm *vm,
>>   				mem_type =
>> region_to_mem_type[region];
>>   			}
>>   
>> -			err = xe_bo_migrate(bo, mem_type, NULL,
>> exec);
>> +			if (mem_type == XE_PL_TT)
>> +				err = xe_bo_migrate_tt_sticky(bo,
>> NULL, exec);
>> +			else
>> +				err = xe_bo_migrate(bo, mem_type,
>> NULL, exec);
>>   		}
>>   		break;
>>   	}