On 29/03/18 05:44 AM, Christian König wrote: > Am 28.03.2018 um 21:53 schrieb Logan Gunthorpe: >> >> On 28/03/18 01:44 PM, Christian König wrote: >>> Well, isn't that exactly what dma_map_resource() is good for? As far as >>> I can see it makes sure IOMMU is aware of the access route and >>> translates a CPU address into a PCI Bus address. >>> I'm using that with the AMD IOMMU driver and at least there it works >>> perfectly fine. >> Yes, it would be nice, but no arch has implemented this yet. We are just >> lucky in the x86 case because that arch is simple and doesn't need to do >> anything for P2P (partially due to the Bus and CPU addresses being the >> same). But in the general case, you can't rely on it. > > Well, that an arch hasn't implemented it doesn't mean that we don't have > the right interface to do it. Yes, but right now we don't have a performant way to check if we are doing P2P or not in the dma_map_X() wrappers. And this is necessary to check if the DMA ops in use support it or not. We can't have the dma_map_X() functions do the wrong thing because they don't support it yet. > Devices integrated in the CPU usually only "claim" to be PCIe devices. > In reality their memory request path go directly through the integrated > north bridge. The reason for this is simple better throughput/latency. These are just more reasons why our patchset restricts to devices behind a switch. And more mess for someone to deal with if they need to relax that restriction. Logan
