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

> > > > > Maybe I just don't quite get the relationship between Root > > > > > Complexes and Root Buses, but I guess my question is: what > > > > > is preventing us from simply doing whatever a > > > > > spapr-pci-host-bridge is doing in order to expose a legacy > > > > > PCI Root Bus (pci.*) to the guest, and create a new > > > > > spapr-pcie-host-bridge that exposes a PCIe Root Bus (pcie.*) > > > > > instead? > > > >  > > > > Hrm, the suggestion of providing both a vanilla-PCI and PCI-E host > > > > bridge came up before.  I think one of us spotted a problem with that, > > > > but I don't recall what it was now.  I guess one is how libvirt would > > > > map it's stupid-fake-domain-numbers to which root bus to use. > >  > > This would be a weird configuration, I never heard of something like that > > on a bare metal machine, but I never worked on pseries, who knows... >  > Which aspect?  Having multiple independent host bridges is perfectly > reasonable - x86 just doesn't do it well for rather stupid historical > reasons.   I agree about the multiple host-bridges, is actually what pxb/pxb-pcie devices (kind of) do.   I was talking about having one PCI PHB and another PHB which is PCI Express. I was referring to one system having both PCI and PCIe PHBs.

> Again, one possible option here is to continue to treat pseries as > having a vanilla-PCI bus, but with a special flag saying that it's > magically able to connect PCI-E devices.   A PCIe bus supporting PCI devices is strange (QEMU allows it ...), but a PCI bus supporting PCIe devices is hard to "swallow".

I would say maybe make it a special case of a PCIe bus with different rules. It can derive from the PCIe bus class and override the usual behavior with PAPR specific rule which happen to be similar with the PCI bus rules.