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

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Message-ID: <a843a970-afec-6541-c54b-4bc2a3c4750a@jonmasters.org>
Date:   Mon, 25 Jan 2021 00:26:52 -0500
From:   Jon Masters <jcm@...masters.org>
To:     Florian Fainelli <f.fainelli@...il.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Claire Chang <tientzu@...omium.org>
Cc:     heikki.krogerus@...ux.intel.com, peterz@...radead.org,
        grant.likely@....com, paulus@...ba.org,
        Frank Rowand <frowand.list@...il.com>, mingo@...nel.org,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        sstabellini@...nel.org, Saravana Kannan <saravanak@...gle.com>,
        Joerg Roedel <joro@...tes.org>, rafael.j.wysocki@...el.com,
        Christoph Hellwig <hch@....de>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        xen-devel@...ts.xenproject.org,
        Thierry Reding <treding@...dia.com>,
        linux-devicetree <devicetree@...r.kernel.org>, will@...nel.org,
        dan.j.williams@...el.com, linuxppc-dev@...ts.ozlabs.org,
        Rob Herring <robh+dt@...nel.org>, boris.ostrovsky@...cle.com,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        jgross@...e.com, Nicolas Boichat <drinkcat@...omium.org>,
        Greg KH <gregkh@...uxfoundation.org>, rdunlap@...radead.org,
        lkml <linux-kernel@...r.kernel.org>,
        Tomasz Figa <tfiga@...omium.org>,
        iommu@...ts.linux-foundation.org, xypron.glpk@....de,
        Robin Murphy <robin.murphy@....com>, bauerman@...ux.ibm.com
Subject: 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
> 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