[RFC PATCH] iommu/arm-smmu-v3: Allow nested attach for PCI bridges without vDEVICE

[Dmitry Malkin]

> This kind of situation is very challenging to setup in the VM, and at
> this point you may as well include the switch to the VM too.

Passing through the PCI switch would not help because the device without
a vDEVICE is the host Root Port. Neither the Root Port nor the switch
should be exposed to the VM.

A multi-device IOMMU group is an intentional effect of using
'pci=config_acs', and I would prefer to restore support for this
configuration upstream.

Dmitry