linux-kernel - Re: [RFC PATCH 1/3] content: Add VIRTIO_F_SWIOTLB to negotiate use of SWIOTLB bounce buffers

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Message-ID: <20250402111901-mutt-send-email-mst@kernel.org>
Date: Wed, 2 Apr 2025 11:20:19 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: David Woodhouse <dwmw2@...radead.org>
Cc: virtio-comment@...ts.linux.dev, hch@...radead.org,
	Claire Chang <tientzu@...omium.org>,
	linux-devicetree <devicetree@...r.kernel.org>,
	Rob Herring <robh+dt@...nel.org>,
	Jörg Roedel <joro@...tes.org>,
	iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	graf@...zon.de
Subject: Re: [RFC PATCH 1/3] content: Add VIRTIO_F_SWIOTLB to negotiate use
 of SWIOTLB bounce buffers

On Wed, Apr 02, 2025 at 04:12:39PM +0100, David Woodhouse wrote:
> On Wed, 2025-04-02 at 10:54 -0400, Michael S. Tsirkin wrote:
> > > +  If a the device transport provides a software IOTLB bounce buffer,
> > > +  addresses within its range are not subject to the requirements of
> > > +  VIRTIO_F_ACCESS_PLATFORM as they are considered to be ``on-device''.
> > 
> > I don't get this part. the system designers currently have a choice
> > whether to have these controlled by VIRTIO_F_ACCESS_PLATFORM or not.
> > with PCI, for example, BAR on the same device is naturally not
> > behind an iommu.
> 
> In the PCI case this *is* a BAR on the same device, and is naturally
> not behind an IOMMU as you say. This is just stating the obvious, for
> clarity.

Then the platform already does this right, and it's better not to
try and override it in the spec.

> For virtio-mmio it also isn't translated by an IOMMU; that was the
> *point* of the `restricted-dma-pool` support.
> 

Clear VIRTIO_F_ACCESS_PLATFORM then?

Generally, it is preferable to keep all features orthogonal if
at all possible.