On Wed, Jun 23, 2021 at 10:00:29PM +0300, Oded Gabbay wrote: > On Wed, Jun 23, 2021 at 9:50 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote: > > > > On Wed, Jun 23, 2021 at 09:43:04PM +0300, Oded Gabbay wrote: > > > > > Can you please explain why it is so important to (allow) access them > > > through the CPU ? > > > > It is not so much important, as it reflects significant design choices > > that are already tightly baked into alot of our stacks. > > > > A SGL is CPU accessible by design - that is baked into this thing and > > places all over the place assume it. Even in RDMA we have > > RXE/SWI/HFI1/qib that might want to use the CPU side (grep for sg_page > > to see) > > > > So, the thing at the top of the stack - in this case the gaudi driver > > - simply can't assume what the rest of the stack is going to do and > > omit the CPU side. It breaks everything. > > > > Logan's patch series is the most fully developed way out of this > > predicament so far. > > I understand the argument and I agree that for the generic case, the > top of the stack can't assume anything. > Having said that, in this case the SGL is encapsulated inside a dma-buf object. > > Maybe its a stupid/over-simplified suggestion, but can't we add a > property to the dma-buf object, > that will be set by the exporter, which will "tell" the importer it > can't use any CPU fallback ? Only "real" p2p ? The block stack has been trying to do something like this. The flag doesn't solve the DMA API/IOMMU problems though. Jason
