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

[Jason Gunthorpe]

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 think I saw EFA's current out of tree implementations had this bug.

> to do is mmap revoke), and I think that model of exclusive device
> ownership with the option to revoke fits pretty well for at least some
> of the accelerators floating around. In that case importers would
> never get a move_notify (maybe we should call this revoke_notify to
> make it clear it's a bit different) callback, except when the entire
> thing has been yanked. I think that would fit pretty well for VFIO,
> and I think we should be able to make it work for rdma too as some
> kind of auto-deregister. The locking might be fun with both of these
> since I expect some inversions compared to the register path, we'll
> have to figure these out.

It fits semantically nicely, VFIO also has a revoke semantic for BAR
mappings.

The challenge is the RDMA side which doesn't have a 'dma disabled
error state' for objects as part of the spec.

Some HW, like mlx5, can implement this for MR objects (see revoke_mr),
but I don't know if anything else can, and even mlx5 currently can't
do a revoke for any other object type.

I don't know how useful it would be, need to check on some of the use
cases.

The locking is tricky as we have to issue a device command, but that
device command cannot run concurrently with destruction or the tail
part of creation.

Jason