IOMMU Archive on
 help / color / Atom feed
From: Alexander Graf via iommu <>
To: Christoph Hellwig <>
Cc: Mark Rutland <>,, Konrad Rzeszutek Wilk <>,, Jan Kiszka <>,,,,,,,,
	Robin Murphy <>,
Subject: Re: [PATCH] swiotlb: Allow swiotlb to live at pre-defined address
Date: Thu, 26 Mar 2020 18:11:31 +0100
Message-ID: <> (raw)
In-Reply-To: <>

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 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.  

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=force" basically allows you to steer the DMA bounce buffering 
decisions already.



Amazon Development Center Germany GmbH
Krausenstr. 38
10117 Berlin
Geschaeftsfuehrung: Christian Schlaeger, Jonathan Weiss
Eingetragen am Amtsgericht Charlottenburg unter HRB 149173 B
Sitz: Berlin
Ust-ID: DE 289 237 879

iommu mailing list

  reply index

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
2020-03-26 17:11   ` Alexander Graf via iommu [this message]
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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

IOMMU Archive on

Archives are clonable:
	git clone --mirror linux-iommu/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-iommu linux-iommu/ \
	public-inbox-index linux-iommu

Example config snippet for mirrors

Newsgroup available over NNTP:

AGPL code for this site: git clone