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Subject: Re: [Qemu-devel] [Qemu-ppc] [RFC PATCH qemu] spapr_pci: Create
 PCI-express root bus by default
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To: David Gibson <david@gibson.dropbear.id.au>, Andrea Bolognani <abologna@redhat.com>
Cc: Alexey Kardashevskiy <aik@ozlabs.ru>, Greg Kurz <groug@kaod.org>, Paolo Bonzini <pbonzini@redhat.com>, Alex Williamson <alex.williamson@redhat.com>, qemu-ppc@nongnu.org, qemu-devel@nongnu.org, libvir-list@redhat.com, Michael Roth <mdroth@linux.vnet.ibm.com>

On Fri, 2016-12-02 at 15:18 +1100, David Gibson wrote:
> But if you pass through multiple groups, things get weird.=C2=A0 On q35=
,
> you'd generally expect physically separate (different slot) devices to
> appear under separate root complexes.=C2=A0 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.=C2=A0 But I suspect that's likely to ca=
use
> trouble with hotplug, and it will certainly need different address
> allocation from libvirt.