From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 21DBFC43387 for ; Thu, 17 Jan 2019 09:30:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E6CE820851 for ; Thu, 17 Jan 2019 09:30:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728674AbfAQJaE (ORCPT ); Thu, 17 Jan 2019 04:30:04 -0500 Received: from verein.lst.de ([213.95.11.211]:36339 "EHLO newverein.lst.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727194AbfAQJaD (ORCPT ); Thu, 17 Jan 2019 04:30:03 -0500 Received: by newverein.lst.de (Postfix, from userid 2407) id 2C7566F9C5; Thu, 17 Jan 2019 10:30:02 +0100 (CET) Date: Thu, 17 Jan 2019 10:30:01 +0100 From: "hch@lst.de" To: Jason Gunthorpe Cc: "hch@lst.de" , 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" Subject: Re: [PATCH] lib/scatterlist: Provide a DMA page iterator Message-ID: <20190117093001.GB31303@lst.de> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190116172436.GM22045@ziepe.ca> User-Agent: Mutt/1.5.17 (2007-11-01) Sender: linux-media-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-media@vger.kernel.org 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.