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Re: [RFC PATCH v3 2/6] swiotlb: Add restricted DMA pool

On 1/7/21 1:09 PM, Florian Fainelli wrote:
On 1/7/21 9:57 AM, Konrad Rzeszutek Wilk wrote:
On Fri, Jan 08, 2021 at 01:39:18AM +0800, Claire Chang wrote:
Hi Greg and Konrad,

This change is intended to be non-arch specific. Any arch that lacks DMA access
control and has devices not behind an IOMMU can make use of it. Could you share
why you think this should be arch specific?

The idea behind non-arch specific code is it to be generic. The devicetree
is specific to PowerPC, Sparc, and ARM, and not to x86 - hence it should
be in arch specific code.

In premise the same code could be used with an ACPI enabled system with
an appropriate service to identify the restricted DMA regions and unlock

More than 1 architecture requiring this function (ARM and ARM64 are the
two I can think of needing this immediately) sort of calls for making
the code architecture agnostic since past 2, you need something that scales.

There is already code today under kernel/dma/contiguous.c that is only
activated on a CONFIG_OF=y && CONFIG_OF_RESERVED_MEM=y system, this is
no different.

<unrelated to these patches, which are useful for the case cited>

Just a note for history/archives that this approach would not be appropriate on general purpose Arm systems, such as SystemReady-ES edge/non-server platforms seeking to run general purpose distros. I want to have that in the record before someone at Arm (or NVidia, or a bunch of others that come to mind who have memory firewalls) gets an idea.

If you're working at an Arm vendor and come looking at this later thinking "wow, what a great idea!", please fix your hardware to have a real IOMMU/SMMU and real PCIe. You'll be pointed at this reply.


Computer Architect



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