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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 36EFAC2D0EC for ; Thu, 26 Mar 2020 17:05:21 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 198E520714 for ; Thu, 26 Mar 2020 17:05:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727829AbgCZRFU (ORCPT ); Thu, 26 Mar 2020 13:05:20 -0400 Received: from verein.lst.de ([213.95.11.211]:46743 "EHLO verein.lst.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726954AbgCZRFT (ORCPT ); Thu, 26 Mar 2020 13:05:19 -0400 Received: by verein.lst.de (Postfix, from userid 2407) id CF3D2227A81; Thu, 26 Mar 2020 18:05:16 +0100 (CET) Date: Thu, 26 Mar 2020 18:05:16 +0100 From: Christoph Hellwig To: Alexander Graf Cc: iommu@lists.linux-foundation.org, Konrad Rzeszutek Wilk , x86@kernel.org, Christoph Hellwig , Marek Szyprowski , Robin Murphy , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, Mark Rutland , dwmw@amazon.com, benh@amazon.com, Jan Kiszka , alcioa@amazon.com, aggh@amazon.com, aagch@amazon.com, dhr@amazon.com Subject: Re: [PATCH] swiotlb: Allow swiotlb to live at pre-defined address Message-ID: <20200326170516.GB6387@lst.de> References: <20200326162922.27085-1-graf@amazon.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200326162922.27085-1-graf@amazon.com> User-Agent: Mutt/1.5.17 (2007-11-01) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Mar 26, 2020 at 05:29:22PM +0100, Alexander Graf wrote: > The swiotlb is a very convenient fallback mechanism for bounce buffering of > DMAable data. It is usually used for the compatibility case where devices > can only DMA to a "low region". > > However, in some scenarios this "low region" may be bound even more > heavily. For example, there are embedded system where only an SRAM region > is shared between device and CPU. There are also heterogeneous computing > scenarios where only a subset of RAM is cache coherent between the > components of the system. There are partitioning hypervisors, where > a "control VM" that implements device emulation has limited view into a > partition's memory for DMA capabilities due to safety concerns. > > This patch adds a command line driven mechanism to move all DMA memory into > a predefined shared memory region which may or may not be part of the > physical address layout of the Operating System. > > Ideally, the typical path to set this configuration would be through Device > Tree or ACPI, but neither of the two mechanisms is standardized yet. Also, > in the x86 MicroVM use case, we have neither ACPI nor Device Tree, but > instead configure the system purely through kernel command line options. > > I'm sure other people will find the functionality useful going forward > though and extend it to be triggered by DT/ACPI in the future. I'm totally against hacking in a kernel parameter for this. We'll need a proper documented DT or ACPI way. We also need to feed this information into the actual DMA bounce buffering decisions and not just the swiotlb placement.