On Thu, Jan 17, 2019 at 10:30:01AM +0100, hch@xxxxxx wrote: > 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. At least on the ARM/MIPS systems I've worked with like this the cache manipulation instructions are privileged and cannot be executed by userspace. So the general case requires a syscall. > 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. I doubt there is any support for that idea... Jason
