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

[Jon Masters <jcm@xxxxxxxxxxxxxx>]

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
> them.
>
> 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.
Jon.

--
Computer Architect