[Intel-gfx] [PATCH] drm/i915/gem: Allow importing of shmemfs objects into any device

[Tvrtko Ursulin]

On 21/01/2021 11:03, Chris Wilson wrote:
> Quoting Tvrtko Ursulin (2021-01-21 11:00:25)
>>
>> On 20/01/2021 20:59, Chris Wilson wrote:
>>> Quoting Chris Wilson (2021-01-20 18:06:08)
>>>> Quoting Matthew Auld (2021-01-20 17:46:10)
>>>>> On Wed, 20 Jan 2021 at 15:40, Chris Wilson <chris at chris-wilson.co.uk> wrote:
>>>>>>
>>>>>> If we import a shmemfs object between devices, for example from
>>>>>> Tigerlake to DG1, we can simply reuse the native object and its backing
>>>>>> store.
>>>>>
>>>>> Hmmm interesting, so does that include re-using the actual sg mapping
>>>>> for the backing pages? Does that work out-of-the-box between different
>>>>> devices assuming we have iommu enabled?
>>>>
>>>> Indeed interesting; the dma_addr_t are supposed to be local to a device.
>>>
>>> On reflection, we are expected to use cross-device dma_addr_t with
>>> dma-buf. It's the exporter who assigns the dma_addr_t for the importer
>>> to use, and they are always given from the original device.
>>>
>>> Maybe not so bad. Definitely needs testing to see what happens in
>>> practice.
>>
>> What about object migration? I did not spot anything preventing it once
>> object was exported like this so owning device could move it to device
>> memory afterwards which would probably be bad.
> 
> Depends on how you do your migration, but your migration should be
> checking that it is allowed to migrate the object first.

Okay agreed, meaning to be handled once migration gets to upstream.

Regards,

Tvrtko