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

[Xiong, Jianxin]

> -----Original Message-----
> From: Jason Gunthorpe <jgg at ziepe.ca>
> Sent: Tuesday, June 30, 2020 12:17 PM
> 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>; dri-
> devel at lists.freedesktop.org
> 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.

Hmm. I thought the pfn corresponds to the address in the BAR range. I could be
wrong here. 

> 
> > > 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..

Yes, this feedback is very helpful. Will work on improving the code.

> 
> > > 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.

Good point. Thanks.

> 
> Jason