On Fri, May 31, 2019 at 12:13:47PM +0100, Jean-Philippe Brucker wrote:

On 30/05/2019 18:45, Michael S. Tsirkin wrote:

quoted

On Thu, May 30, 2019 at 06:09:24PM +0100, Jean-Philippe Brucker wrote:

quoted

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 <redacted>

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.

quoted

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

DECLARE_PCI_FIXUP_EARLY
or
DECLARE_PCI_FIXUP_CLASS_EARLY

in e.g.
arch/x86/kernel/quirks.c
or
drivers/pci/quirks.c

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

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.

-- 
MST