Re: [PATCH v2 18/32] drm/xe/vm: Add attributes struct as member of vma

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

On 15-05-2025 00:06, Matthew Brost wrote:
> On Mon, Apr 07, 2025 at 03:47:05PM +0530, Himal Prasad Ghimiray wrote:
>> The attribute of xe_vma will determine the migration policy and the
>> encoding of the page table entries (PTEs) for that vma.
>> This attribute helps manage how memory pages are moved and how their
>> addresses are translated. It will be used by madvise to set the
>> behavior of the vma.
>>
>> Signed-off-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
>> ---
>>   drivers/gpu/drm/xe/xe_vm.c       |  6 ++++++
>>   drivers/gpu/drm/xe/xe_vm_types.h | 20 ++++++++++++++++++++
>>   2 files changed, 26 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/xe/xe_vm.c b/drivers/gpu/drm/xe/xe_vm.c
>> index 27a8dbe709c2..1ff9e477e061 100644
>> --- a/drivers/gpu/drm/xe/xe_vm.c
>> +++ b/drivers/gpu/drm/xe/xe_vm.c
>> @@ -2470,6 +2470,12 @@ static struct xe_vma *new_vma(struct xe_vm *vm, struct drm_gpuva_op_map *op,
>>   		vma = ERR_PTR(err);
>>   	}
>>   
>> +	/*TODO: assign devmem_fd of local vram once multi device
>> +	 * support is added.
>> +	 */
>> +	vma->attr.preferred_loc.devmem_fd = 1;
> 
> Assigning a value of '1' is a bit odd... I'd prefer using a define or
> something similar to indicate the intended behavior. I noticed a few
> other assignments to '1' in the final result—same comment applies to
> those.

Sure

> 
>> +	vma->attr.atomic_access = DRM_XE_VMA_ATOMIC_UNDEFINED;
>> +
>>   	return vma;
>>   }
>>   
>> diff --git a/drivers/gpu/drm/xe/xe_vm_types.h b/drivers/gpu/drm/xe/xe_vm_types.h
>> index d3c1209348e9..5f5feffecb82 100644
>> --- a/drivers/gpu/drm/xe/xe_vm_types.h
>> +++ b/drivers/gpu/drm/xe/xe_vm_types.h
>> @@ -77,6 +77,19 @@ struct xe_userptr {
>>   #endif
>>   };
>>   
>> +/**
>> + * struct xe_vma_mem_attr - memory attributes associated with vma
>> + */
>> +struct xe_vma_mem_attr {
>> +	/** @preferred_loc: perferred memory_location*/
>> +	struct {
>> +		u32 migration_policy; /* represents migration policies */
>> +		u32 devmem_fd; /* devmem_fd used for determining pagemap_fd requested by user */
>> +	} preferred_loc;
> 
> I'm a little unclear on how these variables work.
> 
> In the uAPI for migration_policy, I see MIGRATE_ALL_PAGES and
> MIGRATE_ONLY_SYSTEM_PAGES (these should probably be normalized with a
> DRM_XE_* prefix, by the way), but it's unclear to me what exactly these
> mean or how they're used based on the final result—could you clarify?

With multi-device support the idea was to have flexibility to move only 
system pages to preferred location or also move pages from other vram 
location to preferred location.

> 
> Likewise, I'm confused about the devmem_fd usage. It can either be
> assigned a devmem_fd from the uAPI, but in some cases, it's interpreted
> as a region. I assume this is anticipating multi-GPU support, but again,
> the plan isn't clear to me. Could you explain?

The devmem_fd is intended to be used to determine the struct drm_pagemap 
*, which in turn will be used to identify the tile associated with VRAM 
for allocation and binding. The changes that introduce the 
devmem_fd->drm_pagemap->tile [1] linkage will be part of the upcoming 
multi-GPU support.

To ensure that the current changes are easily scalable and can be 
extended for multi-GPU support, I am defining devmem_fd in the UAPI and 
using it in the KMD as a region placeholder until multi-GPU support is 
integrated.

[1] https://patchwork.freedesktop.org/patch/642773/?series=146227&rev=1


> 
> In general I agree with the idea of xe_vma_mem_attr though.
> 
> Matt
> 
>> +	/** @atomic_access: The atomic access type for the vma */
>> +	u32 atomic_access;
>> +};
>> +
>>   struct xe_vma {
>>   	/** @gpuva: Base GPUVA object */
>>   	struct drm_gpuva gpuva;
>> @@ -128,6 +141,13 @@ struct xe_vma {
>>   	 * Needs to be signalled before UNMAP can be processed.
>>   	 */
>>   	struct xe_user_fence *ufence;
>> +
>> +	/**
>> +	 * @attr: The attributes of vma which determines the migration policy
>> +	 * and encoding of the PTEs for this vma.
>> +	 */
>> +	struct xe_vma_mem_attr attr;
>> +
>>   };
>>   
>>   /**
>> -- 
>> 2.34.1
>>