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
> >> Reviewed-by: Eric Auger
> >> Signed-off-by: Jean-Philippe Brucker
> >
> > 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
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
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