Since we will not use all 256 buses of an extra PCI domain, I think this space will allow us to support more PCI domains.
Depends on the use case I guess. If you just need many pcie devices this probably doesn't help. If you want them for numa support then yes, more domains with less devices each can be useful then.
How will the flow look like ?
- QEMU passes to SeaBIOS information of how many extra
PCI domains needs, and how many buses per domain. How it will pass this info? A vendor specific capability, some PCI registers or modifying extra-pci-roots fw_cfg file?
Where is the pxb-pcie device? 0000:$somewhere? Or $domain:00:00.0?
- SeaBIOS assigns the address for each PCI Domain and
returns the information to QEMU. How it will do that? Some pxb-pcie registers? Or do we model the MMCFG like a PCI BAR?
If we can access pxb-pcie registers before configuring MMCFG then yes, we should use pxb-pcie registers for that.