Re: [Qemu-devel] [Qemu-ppc] [RFC PATCH qemu] spapr_pci: Create PCI-express root bus by default

[Benjamin Herrenschmidt]

On Fri, 2016-12-02 at 15:18 +1100, David Gibson wrote:
> But if you pass through multiple groups, things get weird.  On q35,
> you'd generally expect physically separate (different slot) devices to
> appear under separate root complexes.  Whereas on pseries they'll
> appear as siblings on a virtual bus (which makes no physical sense for
> point-to-point PCI-E).

It's also somewhat broken if they aren't in the same iommu domain
because the way we pass the iommu buid is via the parent node, so
a given iommu domain must reside below a common parent and not share
it.

> I suppose we could try treating all devices on pseries as though they
> were chipset builtin devices on q35, which will appear on the root
> PCI-E bus without root complex.  But I suspect that's likely to cause
> trouble with hotplug, and it will certainly need different address
> allocation from libvirt.