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

From: John Hubbard
Date: Tue Aug 24 2021 - 13:40:02 EST

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.

John Hubbard