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 2218BC2D0EC for ; Mon, 30 Mar 2020 13:24:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 03FC320733 for ; Mon, 30 Mar 2020 13:24:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728689AbgC3NYZ (ORCPT ); Mon, 30 Mar 2020 09:24:25 -0400 Received: from foss.arm.com ([217.140.110.172]:53574 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727745AbgC3NYY (ORCPT ); Mon, 30 Mar 2020 09:24:24 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id C089D30E; Mon, 30 Mar 2020 06:24:23 -0700 (PDT) Received: from lakrids.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id C30CF3F71E; Mon, 30 Mar 2020 06:24:21 -0700 (PDT) Date: Mon, 30 Mar 2020 14:24:16 +0100 From: Mark Rutland To: Alexander Graf Cc: Christoph Hellwig , iommu@lists.linux-foundation.org, Konrad Rzeszutek Wilk , x86@kernel.org, Marek Szyprowski , Robin Murphy , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, 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: <20200330132416.GA20969@lakrids.cambridge.arm.com> References: <20200326162922.27085-1-graf@amazon.com> <20200326170516.GB6387@lst.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.11.1+11 (2f07cb52) (2018-12-01) 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 06:11:31PM +0100, Alexander Graf wrote: > On 26.03.20 18:05, Christoph Hellwig wrote: > > > > 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. > > I'm with you on that sentiment, but in the environment I'm currently looking > at, we have neither DT nor ACPI: The kernel gets purely configured via > kernel command line. For other unenumerable artifacts on the system, such as > virtio-mmio platform devices, that works well enough and also basically > "hacks a kernel parameter" to specify the system layout. On the arm64 front, you'd *have* to pass a DT to the kernel (as that's where we get the command line from), and we *only* discover memory from the DT or EFI memory map, so the arguments above aren't generally applicable. You can enumerate virtio-mmio devices from DT, also. Device-specific constraints on memory should really be described in a per-device fashion in the FW tables so that the OS can decide how to handle them. Just becuase one device can only access memory in a specific 1MiB window doesn't mean all other should be forced to share the same constraint. I think that's what Christoph was alluding to. Thanks, Mark.