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Subject: Re: [PATCH] swiotlb: Allow swiotlb to live at pre-defined address
To:     Christoph Hellwig <hch@lst.de>
CC:     <iommu@lists.linux-foundation.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
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        <dhr@amazon.com>
References: <20200326162922.27085-1-graf@amazon.com>
 <20200326170516.GB6387@lst.de>
From:   Alexander Graf <graf@amazon.com>
Message-ID: <cef4f2f5-3530-82f8-c0f5-ee0c2701ce6a@amazon.com>
Date:   Thu, 26 Mar 2020 18:11:31 +0100
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On 26.03.20 18:05, Christoph Hellwig wrote:
> =

> 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 i=
nto
>> 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 Dev=
ice
>> Tree or ACPI, but neither of the two mechanisms is standardized yet. Als=
o,
>> 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.  =


I'm with you on that sentiment, but in the environment I'm currently =

looking at, we have neither DT nor ACPI: The kernel gets purely =

configured via kernel command line. For other unenumerable artifacts on =

the system, such as virtio-mmio platform devices, that works well enough =

and also basically "hacks a kernel parameter" to specify the system layout.

> We also need to feed this information
> into the actual DMA bounce buffering decisions and not just the swiotlb
> placement.

Care to elaborate a bit here? I was under the impression that =

"swiotlb=3Dforce" basically allows you to steer the DMA bounce buffering =

decisions already.


Thanks!

Alex


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