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From: Christoph Hellwig <hch@lst.de>
To: Alexander Graf <graf@amazon.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
	benh@amazon.com, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	Jan Kiszka <jan.kiszka@siemens.com>,
	x86@kernel.org, linux-doc@vger.kernel.org,
	linux-kernel@vger.kernel.org, aggh@amazon.com, alcioa@amazon.com,
	iommu@lists.linux-foundation.org, aagch@amazon.com,
	dhr@amazon.com, dwmw@amazon.com,
	Robin Murphy <robin.murphy@arm.com>,
	Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH] swiotlb: Allow swiotlb to live at pre-defined address
Date: Thu, 26 Mar 2020 18:05:16 +0100
Message-ID: <20200326170516.GB6387@lst.de> (raw)
In-Reply-To: <20200326162922.27085-1-graf@amazon.com>

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.
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Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-26 16:29 Alexander Graf via iommu
2020-03-26 17:05 ` Christoph Hellwig [this message]
2020-03-26 17:11   ` Alexander Graf via iommu
2020-03-26 17:16     ` David Woodhouse
2020-03-30 13:24     ` Mark Rutland
2020-03-27  9:58 ` Jan Kiszka
2020-03-28 11:57 ` Dave Young
2020-03-30  6:06   ` Kairui Song
2020-03-30 13:40     ` Konrad Rzeszutek Wilk
2020-03-30 20:42       ` Alexander Graf via iommu
2020-03-30 23:37         ` Anthony Yznaga
2020-03-31  1:59         ` Dave Young
2020-03-31  2:16         ` Baoquan He
2020-03-31  1:46       ` Dave Young

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