[RFC PATCH v2 0/3] RDMA: add dma-buf support

[Jason Gunthorpe]

On Wed, Jul 01, 2020 at 11:03:06AM +0200, Christian König wrote:
> Am 30.06.20 um 20:46 schrieb Xiong, Jianxin:
> > > From: Jason Gunthorpe <jgg at ziepe.ca>
> > > Sent: Tuesday, June 30, 2020 10:35 AM
> > > To: Xiong, Jianxin <jianxin.xiong at intel.com>
> > > Cc: linux-rdma at vger.kernel.org; Doug Ledford <dledford at redhat.com>; Sumit Semwal <sumit.semwal at linaro.org>; Leon Romanovsky
> > > <leon at kernel.org>; Vetter, Daniel <daniel.vetter at intel.com>; Christian Koenig <christian.koenig at amd.com>
> > > Subject: Re: [RFC PATCH v2 0/3] RDMA: add dma-buf support
> > > 
> > > On Tue, Jun 30, 2020 at 05:21:33PM +0000, Xiong, Jianxin wrote:
> > > > > > Heterogeneous Memory Management (HMM) utilizes
> > > > > > mmu_interval_notifier and ZONE_DEVICE to support shared virtual
> > > > > > address space and page migration between system memory and device
> > > > > > memory. HMM doesn't support pinning device memory because pages
> > > > > > located on device must be able to migrate to system memory when
> > > > > > accessed by CPU. Peer-to-peer access is possible if the peer can
> > > > > > handle page fault. For RDMA, that means the NIC must support on-demand paging.
> > > > > peer-peer access is currently not possible with hmm_range_fault().
> > > > Currently hmm_range_fault() always sets the cpu access flag and device
> > > > private pages are migrated to the system RAM in the fault handler.
> > > > However, it's possible to have a modified code flow to keep the device
> > > > private page info for use with peer to peer access.
> > > Sort of, but only within the same device, RDMA or anything else generic can't reach inside a DEVICE_PRIVATE and extract anything useful.
> > But pfn is supposed to be all that is needed.
> > 
> > > > > So.. this patch doesn't really do anything new? We could just make a MR against the DMA buf mmap and get to the same place?
> > > > That's right, the patch alone is just half of the story. The
> > > > functionality depends on availability of dma-buf exporter that can pin
> > > > the device memory.
> > > Well, what do you want to happen here? The RDMA parts are reasonable, but I don't want to add new functionality without a purpose - the
> > > other parts need to be settled out first.
> > At the RDMA side, we mainly want to check if the changes are acceptable. For example,
> > the part about adding 'fd' to the device ops and the ioctl interface. All the previous
> > comments are very helpful for us to refine the patch so that we can be ready when
> > GPU side support becomes available.
> > 
> > > The need for the dynamic mapping support for even the current DMA Buf hacky P2P users is really too bad. Can you get any GPU driver to
> > > support non-dynamic mapping?
> > We are working on direct direction.
> > 
> > > > > > migrate to system RAM. This is due to the lack of knowledge about
> > > > > > whether the importer can perform peer-to-peer access and the lack
> > > > > > of resource limit control measure for GPU. For the first part, the
> > > > > > latest dma-buf driver has a peer-to-peer flag for the importer,
> > > > > > but the flag is currently tied to dynamic mapping support, which
> > > > > > requires on-demand paging support from the NIC to work.
> > > > > ODP for DMA buf?
> > > > Right.
> > > Hum. This is not actually so hard to do. The whole dma buf proposal would make a lot more sense if the 'dma buf MR' had to be the
> > > dynamic kind and the driver had to provide the faulting. It would not be so hard to change mlx5 to be able to work like this, perhaps. (the
> > > locking might be a bit tricky though)
> > The main issue is that not all NICs support ODP.
> 
> You don't need on-demand paging support from the NIC for dynamic mapping to
> work.
> 
> All you need is the ability to stop wait for ongoing accesses to end and
> make sure that new ones grab a new mapping.

Swap and flush isn't a general HW ability either..

I'm unclear how this could be useful, it is guarenteed to corrupt
in-progress writes?

Did you mean pause, swap and resume? That's ODP.

Jason