On 20/08/2021 15:33, Jason Gunthorpe wrote: > On Fri, Aug 20, 2021 at 09:25:30AM +0200, Daniel Vetter wrote: >> On Fri, Aug 20, 2021 at 1:06 AM Jason Gunthorpe <jgg@xxxxxxxx> wrote: >>> 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@xxxxxxxxxx> 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@xxxxxxxxxx> >>>> >>>> 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. >> >> Oh sure, I've been having these for a while. I think there's two options: >> - some kind of soft-pin, where the contract is that we only revoke >> when absolutely necessary, and it's expected to be catastrophic on the >> importer's side. > > Honestly, I'm not very keen on this. We don't really have HW support > in several RDMA scenarios for even catastrophic unpin. > > Gal, can EFA even do this for a MR? You basically have to resize the > rkey/lkey to zero length (or invalidate it like a FMR) under the > catstrophic revoke. The rkey/lkey cannot just be destroyed as that > opens a security problem with rkey/lkey re-use. I had some discussions with our hardware guys about such functionality in the past, I think it should be doable (not necessarily the same way that FMR does it). Though it would've been nicer if we could agree on a solution that could work for more than 1-2 RDMA devices, using the existing tools the RDMA subsystem has. That's why I tried to approach this by denying such attachments for non-ODP importers instead of exposing a "limited" dynamic importer.
