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Subject: Re: [virtio-dev] Re: [PATCH v8 2/7] dt-bindings: virtio: Add virtio-pci-iommu node

On Fri, May 31, 2019 at 12:13:47PM +0100, Jean-Philippe Brucker wrote:
> On 30/05/2019 18:45, Michael S. Tsirkin wrote:
> > On Thu, May 30, 2019 at 06:09:24PM +0100, Jean-Philippe Brucker wrote:
> >> Some systems implement virtio-iommu as a PCI endpoint. The operating
> >> system needs to discover the relationship between IOMMU and masters long
> >> before the PCI endpoint gets probed. Add a PCI child node to describe the
> >> virtio-iommu device.
> >>
> >> The virtio-pci-iommu is conceptually split between a PCI programming
> >> interface and a translation component on the parent bus. The latter
> >> doesn't have a node in the device tree. The virtio-pci-iommu node
> >> describes both, by linking the PCI endpoint to "iommus" property of DMA
> >> master nodes and to "iommu-map" properties of bus nodes.
> >>
> >> Reviewed-by: Rob Herring <robh@kernel.org>
> >> Reviewed-by: Eric Auger <eric.auger@redhat.com>
> >> Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
> > 
> > So this is just an example right?
> > We are not defining any new properties or anything like that.
> Yes it's just an example. The properties already exist but it's good to
> describe how to put them together for this particular case, because
> there isn't a precedent describing the topology for an IOMMU that
> appears on the PCI bus.
> > I think down the road for non dt platforms we want to put this
> > info in the config space of the device. I do not think ACPI
> > is the best option for this since not all systems have it.
> > But that can wait.
> There is the probe order problem - PCI needs this info before starting
> to probe devices on the bus.

This isn't all that special - it's pretty common for
IOMMUs to be pci devices. The solution is to have the device on
bus 0. For example, add it with


in e.g.

You can also use the configuration access capability
if there's need to access the device before its memory is

> Maybe we could store the info in a separate
> memory region, that is referenced on the command-line and that the guest
> can read early.
> Thanks,
> Jean

The point is to avoid command line hacks. Devices should be
self describing.


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