IOMMU Archive on lore.kernel.org
 help / color / Atom feed
From: Mark Rutland <mark.rutland@arm.com>
To: Alexander Graf <graf@amazon.com>
Cc: benh@amazon.com, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	Jan Kiszka <jan.kiszka@siemens.com>,
	x86@kernel.org, linux-doc@vger.kernel.org,
	linux-kernel@vger.kernel.org, aggh@amazon.com, alcioa@amazon.com,
	iommu@lists.linux-foundation.org, aagch@amazon.com,
	dhr@amazon.com, dwmw@amazon.com,
	Robin Murphy <robin.murphy@arm.com>,
	Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH] swiotlb: Allow swiotlb to live at pre-defined address
Date: Mon, 30 Mar 2020 14:24:16 +0100
Message-ID: <20200330132416.GA20969@lakrids.cambridge.arm.com> (raw)
In-Reply-To: <cef4f2f5-3530-82f8-c0f5-ee0c2701ce6a@amazon.com>

On Thu, Mar 26, 2020 at 06:11:31PM +0100, Alexander Graf wrote:
> 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.

On the arm64 front, you'd *have* to pass a DT to the kernel (as that's
where we get the command line from), and we *only* discover memory
from the DT or EFI memory map, so the arguments above aren't generally
applicable. You can enumerate virtio-mmio devices from DT, also.

Device-specific constraints on memory should really be described in a
per-device fashion in the FW tables so that the OS can decide how to
handle them. Just becuase one device can only access memory in a
specific 1MiB window doesn't mean all other should be forced to share
the same constraint. I think that's what Christoph was alluding to.

Thanks,
Mark.
_______________________________________________
iommu mailing list
iommu@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/iommu

  parent 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
2020-03-26 17:16     ` David Woodhouse
2020-03-30 13:24     ` Mark Rutland [this message]
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:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

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

  git send-email \
    --in-reply-to=20200330132416.GA20969@lakrids.cambridge.arm.com \
    --to=mark.rutland@arm.com \
    --cc=aagch@amazon.com \
    --cc=aggh@amazon.com \
    --cc=alcioa@amazon.com \
    --cc=benh@amazon.com \
    --cc=dhr@amazon.com \
    --cc=dwmw@amazon.com \
    --cc=graf@amazon.com \
    --cc=hch@lst.de \
    --cc=iommu@lists.linux-foundation.org \
    --cc=jan.kiszka@siemens.com \
    --cc=konrad.wilk@oracle.com \
    --cc=linux-doc@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=robin.murphy@arm.com \
    --cc=x86@kernel.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

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

IOMMU Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/linux-iommu/0 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/ https://lore.kernel.org/linux-iommu \
		iommu@lists.linux-foundation.org
	public-inbox-index linux-iommu

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.linux-foundation.lists.iommu


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git