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Intel Mac OS X 10.14; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 MIME-Version: 1.0 In-Reply-To: <20200326170516.GB6387@lst.de> Content-Language: en-US X-Originating-IP: [10.43.162.39] X-ClientProxiedBy: EX13D36UWA004.ant.amazon.com (10.43.160.175) To EX13D20UWC001.ant.amazon.com (10.43.162.244) Cc: Mark Rutland , benh@amazon.com, Konrad Rzeszutek Wilk , linux-doc@vger.kernel.org, Jan Kiszka , x86@kernel.org, linux-kernel@vger.kernel.org, alcioa@amazon.com, iommu@lists.linux-foundation.org, aagch@amazon.com, dhr@amazon.com, dwmw@amazon.com, Robin Murphy , aggh@amazon.com X-BeenThere: iommu@lists.linux-foundation.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: Development issues for Linux IOMMU support List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , From: Alexander Graf via iommu Reply-To: Alexander Graf Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii"; Format="flowed" Errors-To: iommu-bounces@lists.linux-foundation.org Sender: "iommu" 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. > We also need to feed this information > into the actual DMA bounce buffering decisions and not just the swiotlb > placement. Care to elaborate a bit here? I was under the impression that "swiotlb=force" basically allows you to steer the DMA bounce buffering decisions already. Thanks! Alex Amazon Development Center Germany GmbH Krausenstr. 38 10117 Berlin Geschaeftsfuehrung: Christian Schlaeger, Jonathan Weiss Eingetragen am Amtsgericht Charlottenburg unter HRB 149173 B Sitz: Berlin Ust-ID: DE 289 237 879 _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu