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

[Jason Gunthorpe]

On Wed, Aug 18, 2021 at 11:34:51AM +0200, Daniel Vetter wrote:
> On Wed, Aug 18, 2021 at 9:45 AM Gal Pressman <galpress at amazon.com> wrote:
> >
> > Hey all,
> >
> > Currently, the RDMA subsystem can only work with dynamic dmabuf
> > attachments, which requires the RDMA device to support on-demand-paging
> > (ODP) which is not common on most devices (only supported by mlx5).
> >
> > While the dynamic requirement makes sense for certain GPUs, some devices
> > (such as habanalabs) have device memory that is always "pinned" and do
> > not need/use the move_notify operation.
> >
> > The motivation of this RFC is to use habanalabs as the dmabuf exporter,
> > and EFA as the importer to allow for peer2peer access through libibverbs.
> >
> > This draft patch changes the dmabuf driver to differentiate between
> > static/dynamic attachments by looking at the move_notify op instead of
> > the importer_ops struct, and allowing the peer2peer flag to be enabled
> > in case of a static exporter.
> >
> > Thanks
> >
> > Signed-off-by: Gal Pressman <galpress at amazon.com>
> 
> Given that habanalabs dma-buf support is very firmly in limbo (at
> least it's not yet in linux-next or anywhere else) I think you want to
> solve that problem first before we tackle the additional issue of
> making p2p work without dynamic dma-buf. Without that it just doesn't
> make a lot of sense really to talk about solutions here.

I have been thinking about adding a dmabuf exporter to VFIO, for
basically the same reason habana labs wants to do it.

In that situation we'd want to see an approach similar to this as well
to have a broad usability.

The GPU drivers also want this for certain sophisticated scenarios
with RDMA, the intree drivers just haven't quite got there yet.

So, I think it is worthwhile to start thinking about this regardless
of habana labs.

Jason