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

[Gal Pressman]

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 at ziepe.ca> 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 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.
>>
>> 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.