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@xxxxxxxx> > > > Sent: Tuesday, June 30, 2020 10:35 AM > > > To: Xiong, Jianxin <jianxin.xiong@xxxxxxxxx> > > > Cc: linux-rdma@xxxxxxxxxxxxxxx; Doug Ledford <dledford@xxxxxxxxxx>; Sumit Semwal <sumit.semwal@xxxxxxxxxx>; Leon Romanovsky > > > <leon@xxxxxxxxxx>; Vetter, Daniel <daniel.vetter@xxxxxxxxx>; Christian Koenig <christian.koenig@xxxxxxx> > > > 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
