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

[Jason Gunthorpe]

On Tue, Jun 30, 2020 at 06:46:17PM +0000, Xiong, Jianxin wrote:
> > 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.

Needed for what? The PFN of the DEVICE_PRIVATE pages is useless for
anything.

> > 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.

Well, I'm not totally happy with the way the umem and the fd is
handled so roughly and incompletely..

> > 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.

Sure, but there is lots of infrastructure work here to be done on dma
buf, having a correct consumer in the form of ODP might be helpful to
advance it.

Jason