On 10/19/2016 6:51 AM, Dan Williams wrote: > On Tue, Oct 18, 2016 at 2:42 PM, Stephen Bates <sbates@xxxxxxxxxxxx> wrote: >> 1. Address Translation. Suggestions have been made that in certain >> architectures and topologies the dma_addr_t passed to the DMA master >> in a peer-2-peer transfer will not correctly route to the IO memory >> intended. However in our testing to date we have not seen this to be >> an issue, even in systems with IOMMUs and PCIe switches. It is our >> understanding that an IOMMU only maps system memory and would not >> interfere with device memory regions. I'm not sure that's the case. I think it works because with ZONE_DEVICE, the iommu driver will simply treat a dma_map_page call as any other PFN, and create a mapping as it does for any memory page. >> (It certainly has no opportunity >> to do so if the transfer gets routed through a switch). It can still go through the IOMMU if you enable ACS upstream forwarding. > There may still be platforms where peer-to-peer cycles are routed up > through the root bridge and then back down to target device, but we > can address that when / if it happens. I agree. > I wonder if we could (ab)use a > software-defined 'pasid' as the requester id for a peer-to-peer > mapping that needs address translation. Why would you need that? Isn't it enough to map the peer-to-peer addresses correctly in the iommu driver? Haggai -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>