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Date:   Thu, 26 Mar 2020 18:05:16 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Alexander Graf <graf@amazon.com>
Cc:     iommu@lists.linux-foundation.org,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        x86@kernel.org, Christoph Hellwig <hch@lst.de>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Robin Murphy <robin.murphy@arm.com>, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, Mark Rutland <mark.rutland@arm.com>,
        dwmw@amazon.com, benh@amazon.com,
        Jan Kiszka <jan.kiszka@siemens.com>, alcioa@amazon.com,
        aggh@amazon.com, aagch@amazon.com, dhr@amazon.com
Subject: Re: [PATCH] swiotlb: Allow swiotlb to live at pre-defined address
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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.