On Mon, 11 Jul 2016, Theo de Raadt wrote: > > Openbsd on amd64 assumes that DMA is only possible to the lower 4GB. > > Not exactly. On an architecture-by-architecture basis, OpenBSD is > capable of insisting DMA reachable memory only lands in a smaller zone > of memory -- because it makes the other layers of code easier. > > > More interesting would be bufs and mbufs. > > Why is it interesting for mbufs? Please describe the environment > where anywhere near that many mbufs make sense. > > And bufs don't need it either. Have you actually cranked your buffer > cache that high? I have test this, on sparc64 which has unlimited DMA > reach due to the iommu. The system comes to a crawl when there are > too many mbufs or bufs, probably due to management structures unable > to handle the pressure.
No, I didn't know that. I assumed that having a few more GBs of bufcache would help the performance. Until that is the case, 64bit dma does not make much sense. > > What is the usage case for this diff, if it cannot be enabled? >
