Re: [PATCH v3 19/19] drm/xe/bo: Update atomic_access attribute on madvise

["Ghimiray, Himal Prasad" <himal.prasad.ghimiray@intel.com>]

On 29-05-2025 05:16, Matthew Brost wrote:
> On Tue, May 27, 2025 at 10:10:03PM +0530, Himal Prasad Ghimiray wrote:
>> Update the bo_atomic_access based on user-provided input and determine
>> the migration to smem during a CPU fault
>>
>> v2 (Matthew Brost)
>> - Avoid cpu unmapping if bo is already in smem
>> - check atomics on smem too for ioctl
>> - Add comments
>>
>> Signed-off-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
>> ---
>>   drivers/gpu/drm/xe/xe_bo.c         | 21 ++++++++++++--
>>   drivers/gpu/drm/xe/xe_vm.c         | 11 ++++++--
>>   drivers/gpu/drm/xe/xe_vm_madvise.c | 45 ++++++++++++++++++++++++++++--
>>   3 files changed, 69 insertions(+), 8 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/xe/xe_bo.c b/drivers/gpu/drm/xe/xe_bo.c
>> index d99d91fe8aa9..9072e8ae3f3e 100644
>> --- a/drivers/gpu/drm/xe/xe_bo.c
>> +++ b/drivers/gpu/drm/xe/xe_bo.c
>> @@ -1662,6 +1662,12 @@ static void xe_gem_object_close(struct drm_gem_object *obj,
>>   	}
>>   }
>>   
>> +static bool should_migrate_to_smem(struct xe_bo *bo)
>> +{
> 
> xe_bo_assert_held, more on that in reply to previous patch.

Sure

> 
>> +	return bo->attr.atomic_access == DRM_XE_VMA_ATOMIC_GLOBAL ||
>> +	       bo->attr.atomic_access == DRM_XE_VMA_ATOMIC_CPU;
>> +}
>> +
> 
> Hmm, this is tricky. I guess this means sharded atomics on BOs do not
> just work whereas for SVM they do (i.e., DRM_XE_VMA_ATOMIC_UNDEFINED
> means atomics do not work for BOs but for SVM they do). I suppose this
> is the current behavior. I think this will need to be document in the
> uAPI kernel doc.

Makes sense

> 
>>   static vm_fault_t xe_gem_fault(struct vm_fault *vmf)
>>   {
>>   	struct ttm_buffer_object *tbo = vmf->vma->vm_private_data;
>> @@ -1670,7 +1676,7 @@ static vm_fault_t xe_gem_fault(struct vm_fault *vmf)
>>   	struct xe_bo *bo = ttm_to_xe_bo(tbo);
>>   	bool needs_rpm = bo->flags & XE_BO_FLAG_VRAM_MASK;
>>   	vm_fault_t ret;
>> -	int idx;
>> +	int idx, r = 0;
>>   
>>   	if (needs_rpm)
>>   		xe_pm_runtime_get(xe);
>> @@ -1682,8 +1688,17 @@ static vm_fault_t xe_gem_fault(struct vm_fault *vmf)
>>   	if (drm_dev_enter(ddev, &idx)) {
>>   		trace_xe_bo_cpu_fault(bo);
>>   
>> -		ret = ttm_bo_vm_fault_reserved(vmf, vmf->vma->vm_page_prot,
>> -					       TTM_BO_VM_NUM_PREFAULT);
>> +		if (should_migrate_to_smem(bo)) {
>> +			r = xe_bo_migrate(bo, XE_PL_TT);
>> +			if (r == -EBUSY || r == -ERESTARTSYS || r == -EINTR)
>> +				ret = VM_FAULT_NOPAGE;
>> +			else if (r)
>> +				ret = VM_FAULT_SIGBUS;
>> +		}
>> +		if (!ret)
>> +			ret = ttm_bo_vm_fault_reserved(vmf,
>> +						       vmf->vma->vm_page_prot,
>> +						       TTM_BO_VM_NUM_PREFAULT);
>>   		drm_dev_exit(idx);
>>   	} else {
>>   		ret = ttm_bo_vm_dummy_page(vmf, vmf->vma->vm_page_prot);
>> diff --git a/drivers/gpu/drm/xe/xe_vm.c b/drivers/gpu/drm/xe/xe_vm.c
>> index 9611d7ca2bed..1bdf85c12374 100644
>> --- a/drivers/gpu/drm/xe/xe_vm.c
>> +++ b/drivers/gpu/drm/xe/xe_vm.c
>> @@ -3116,9 +3116,16 @@ static int op_lock_and_prep(struct drm_exec *exec, struct xe_vm *vm,
>>   		err = vma_lock_and_validate(exec,
>>   					    gpuva_to_vma(op->base.prefetch.va),
>>   					    false);
>> -		if (!err && !xe_vma_has_no_bo(vma))
>> -			err = xe_bo_migrate(xe_vma_bo(vma),
>> +		if (!err && !xe_vma_has_no_bo(vma)) {
>> +			struct xe_bo *bo = xe_vma_bo(vma);
>> +
>> +			if (region == 0 && !vm->xe->info.has_device_atomics_on_smem &&
>> +			    bo->attr.atomic_access == DRM_XE_VMA_ATOMIC_DEVICE)
>> +				region = 1;
> 
> I wonder if it better to just leave region as is and let the next atomic
> fault trigger the migration.

Ok. lets do it that way.

> 
>> +
>> +			err = xe_bo_migrate(bo,
>>   					    region_to_mem_type[region]);
>> +		}
>>   		break;
>>   	}
>>   	default:
>> diff --git a/drivers/gpu/drm/xe/xe_vm_madvise.c b/drivers/gpu/drm/xe/xe_vm_madvise.c
>> index 0f0b94cb43f2..e048eb48826c 100644
>> --- a/drivers/gpu/drm/xe/xe_vm_madvise.c
>> +++ b/drivers/gpu/drm/xe/xe_vm_madvise.c
>> @@ -82,15 +82,54 @@ static int madvise_atomic(struct xe_device *xe, struct xe_vm *vm,
>>   			  struct xe_vma **vmas, int num_vmas,
>>   			  struct drm_xe_madvise_ops ops)
>>   {
>> -	int i;
>> +	struct xe_bo *bo;
>> +	int err, i;
>>   
>>   	xe_assert(vm->xe, ops.type == DRM_XE_VMA_ATTR_ATOMIC);
>>   	xe_assert(vm->xe, ops.atomic.val > DRM_XE_VMA_ATOMIC_UNDEFINED &&
>>   		  ops.atomic.val <= DRM_XE_VMA_ATOMIC_CPU);
>>   
> 
> Do you sanitize ops.atomic.val prior to this? Also do we disallow a user
> setting DRM_XE_VMA_ATOMIC_UNDEFINED? If not, then this needs to be >=
> DRM_XE_VMA_ATOMIC_UNDEFINED.
Agreed it should be  >= DRM_XE_VMA_ATOMIC_UNDEFINED. And instead of 
assertion will sanitize it here only.

> 
>> -	for (i = 0; i < num_vmas; i++)
>> +	for (i = 0; i < num_vmas; i++) {
>>   		vmas[i]->attr.atomic_access = ops.atomic.val;
>> -	/*TODO: handle bo backed vmas */
>> +
>> +		bo = xe_vma_bo(vmas[i]);
>> +		if (!bo)
>> +			continue;
>> +
>> +		if (XE_IOCTL_DBG(xe, ops.atomic.val == DRM_XE_VMA_ATOMIC_CPU &&
>> +				 !(bo->flags & XE_BO_FLAG_SYSTEM)))
>> +			return -EINVAL;
>> +
> 
> Note when we fail here (or anywhere else in madvise), we could be in a
> state where madvise has partially completed. I think that is actually ok
> as nothing in madvise is fatal as we are just changing attributes. But I
> think we need to document this in the uAPI kernel doc that if madvise
> fails, the state of madvise attributes are undefined.

Will add in kernel-doc of uAPI.

> 
> In practice this really should never fail unless a user is giving bad
> input or extreme memory pressure and kmalloc fails.
> 
> Matt
> 
>> +		/* NOTE: The following atomic checks are platform-specific. For example,
>> +		 * if a device supports CXL atomics, these may not be necessary or
>> +		 * may behave differently.
>> +		 */
>> +		if (XE_IOCTL_DBG(xe, ops.atomic.val == DRM_XE_VMA_ATOMIC_DEVICE &&
>> +				 !(bo->flags & XE_BO_FLAG_VRAM0) &&
>> +				 !(bo->flags & XE_BO_FLAG_VRAM1) &&
>> +				 !(bo->flags & XE_BO_FLAG_SYSTEM &&
>> +				   xe->info.has_device_atomics_on_smem)))
>> +			return -EINVAL;
>> +
>> +		if (XE_IOCTL_DBG(xe, ops.atomic.val == DRM_XE_VMA_ATOMIC_GLOBAL &&
>> +				 (!(bo->flags & XE_BO_FLAG_SYSTEM) ||
>> +				  (!(bo->flags & XE_BO_FLAG_VRAM0) &&
>> +				   !(bo->flags & XE_BO_FLAG_VRAM1)))))
>> +			return -EINVAL;
>> +
>> +		err = xe_bo_lock(bo, true);
>> +		if (err)
>> +			return err;
>> +		bo->attr.atomic_access = ops.atomic.val;
>> +
>> +		/* Invalidate cpu page table, so bo can migrate to smem in next access */
>> +		if (xe_bo_is_vram(bo) &&
>> +		    (bo->attr.atomic_access == DRM_XE_VMA_ATOMIC_CPU ||
>> +		     bo->attr.atomic_access == DRM_XE_VMA_ATOMIC_GLOBAL))
>> +			ttm_bo_unmap_virtual(&bo->ttm);
>> +
>> +		xe_bo_unlock(bo);
>> +	}
>>   	return 0;
>>   }
>>   
>> -- 
>> 2.34.1
>>