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

[Christian König]

Am 01.09.21 um 13:20 schrieb 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?

I think so, yes.

> If so, do we still want a move_notify callback for non-dynamic importers? A noop?

Well we could make the move_notify callback optional, e.g. so that you 
get the new locking approach but still pin the buffers manually with 
dma_buf_pin().

Regards,
Christian.