From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jason Gunthorpe Subject: Re: [PATCH] lib/scatterlist: Provide a DMA page iterator Date: Thu, 17 Jan 2019 08:54:17 -0700 Message-ID: <20190117155417.GA9629@ziepe.ca> References: <20190104223531.GA1705@ziepe.ca> <20190110234218.GM6890@ziepe.ca> <20190114094856.GB29604@lst.de> <1fb20ab4b171b281e9994b6c55734c120958530b.camel@vmware.com> <20190115212501.GE22045@ziepe.ca> <20190116161134.GA29041@lst.de> <20190116172436.GM22045@ziepe.ca> <20190117093001.GB31303@lst.de> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20190117093001.GB31303@lst.de> Sender: linux-kernel-owner@vger.kernel.org To: "hch@lst.de" Cc: Thomas Hellstrom , "linux-kernel@vger.kernel.org" , "yong.zhi@intel.com" , "daniel.vetter@ffwll.ch" , "linux-rdma@vger.kernel.org" , "syeh@vmware.com" , "linux-media@vger.kernel.org" , "bingbu.cao@intel.com" , "imre.deak@intel.com" , "tian.shu.qiu@intel.com" , "jian.xu.zheng@intel.com" , "shiraz.saleem@intel.com" , "sakari.ailus@linux.intel.com" , "dri-devel@lists.freedesktop.org" List-Id: linux-rdma@vger.kernel.org On Thu, Jan 17, 2019 at 10:30:01AM +0100, hch@lst.de 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