On Wed, Jan 16, 2019 at 10:24:36AM -0700, Jason Gunthorpe wrote: > The fact is there is 0 industry interest in using RDMA on platforms > that can't do HW DMA cache coherency - the kernel syscalls required to > do the cache flushing on the IO path would just destroy performance to > the point of making RDMA pointless. Better to use netdev on those > platforms.
In general there is no syscall required for doing cache flushing, you just issue the proper instructions directly from userspace. > The reality is that *all* the subsytems doing DMA kernel bypass are > ignoring the DMA mapping rules, I think we should support this better, > and just accept that user space DMA will not be using syncing. Block > access in cases when this is required, otherwise let it work as is > today. In that case we just need to block userspace DMA access entirely. Which given the amount of problems it creates sounds like a pretty good idea anyway.
