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

[John Hubbard]

On 8/24/21 10:32 AM, Jason Gunthorpe wrote:
...
>>> 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.
> 

OK. I just want to avoid creating any API-level assumptions that dma_buf_pin()
necessarily implies or requires migrating to host memory.

thanks,
-- 
John Hubbard
NVIDIA