Re: [RFC] Make use of non-dynamic dmabuf in RDMA

[Gal Pressman]

On 24/08/2021 20:32, Jason Gunthorpe wrote:
> On Tue, Aug 24, 2021 at 10:27:23AM -0700, John Hubbard wrote:
>> On 8/24/21 2:32 AM, Christian König wrote:
>>> Am 24.08.21 um 11:06 schrieb Gal Pressman:
>>>> On 23/08/2021 13:43, Christian König wrote:
>>>>> Am 21.08.21 um 11:16 schrieb Gal Pressman:
>>>>>> On 20/08/2021 17:32, Jason Gunthorpe wrote:
>>>>>>> On Fri, Aug 20, 2021 at 03:58:33PM +0300, Gal Pressman wrote:
>> ...
>>>>>> IIUC, we're talking about three different exporter "types":
>>>>>> - Dynamic with move_notify (requires ODP)
>>>>>> - Dynamic with revoke_notify
>>>>>> - Static
>>>>>>
>>>>>> Which changes do we need to make the third one work?
>>>>> Basically none at all in the framework.
>>>>>
>>>>> You just need to properly use the dma_buf_pin() function when you start using a
>>>>> buffer (e.g. before you create an attachment) and the dma_buf_unpin() function
>>>>> after you are done with the DMA-buf.
>>>> I replied to your previous mail, but I'll ask again.
>>>> Doesn't the pin operation migrate the memory to host memory?
>>>
>>> Sorry missed your previous reply.
>>>
>>> And yes at least for the amdgpu driver we migrate the memory to host
>>> memory as soon as it is pinned and I would expect that other GPU drivers
>>> do something similar.
>>
>> Well...for many topologies, migrating to host memory will result in a
>> dramatically slower p2p setup. For that reason, some GPU drivers may
>> want to allow pinning of video memory in some situations.
>>
>> Ideally, you've got modern ODP devices and you don't even need to pin.
>> But if not, and you still hope to do high performance p2p between a GPU
>> and a non-ODP Infiniband device, then you would need to leave the pinned
>> memory in vidmem.
>>
>> So I think we don't want to rule out that behavior, right? Or is the
>> thinking more like, "you're lucky that this old non-ODP setup works at
>> all, and we'll make it work by routing through host/cpu memory, but it
>> will be slow"?
> 
> I think it depends on the user, if the user creates memory which is
> permanently located on the GPU then it should be pinnable in this way
> without force migration. But if the memory is inherently migratable
> then it just cannot be pinned in the GPU at all as we can't
> indefinately block migration from happening eg if the CPU touches it
> later or something.

So are we OK with exporters implementing dma_buf_pin() without migrating the memory?
If so, do we still want a move_notify callback for non-dynamic importers? A noop?