Bug 1410578

By default libvirt will disable IO space support for PCI Express Root Ports since all PCI Express devices should work just fine using only MMIO.

However there are cases when we do want to enable IO space support:
1. We might want to assign a phys device (using vfio) and the device is not spec
   compliant and requires IO ports to function properly.
2. We might have a guest firmware with an ancient driver knowing only IO ports.

Decisions like this cannot be done by libvirt, we need the upper layer to decide which PCI Express Root Ports need to have IO ports enabled.

Marcel can you provide some detail on what the actual use case is driving this?

(In reply to Stephen Gordon from comment #2)
> Marcel can you provide some detail on what the actual use case is driving
> this?

Hi Stephen,
I am sorry for the delayed response, I was in PTO.

The problem is the IO ports space is rather limited and for PCI Express machines each PCI Express Root Port allows only to a single (multi-function) device. Since each such port is actually a PCI bridge, the guest firmware and after that the guest OS will try to allocate some IO range for each.

That means that after ~10 PCI root ports are used all IO ports space is exhausted and then:
 1. The firmware may panic and halt (SeaBIOS does that).
 2. The OS may try to assign the IO space by itself and in the best scenario we only see some warnings on logs.

Since all PCI Express devices should be able to work without using IO ports we can instruct the PCI Express Root Ports to disable IO ports by default.

The interesting part is what happens if we need to plug an assigned device (vfio) to a Root Port and it requires IO ports to function properly?

Since now the Root Ports have IO space disabled by default, we need to 'enable' it by setting the corresponding property.

Thanks,
Marcel

There's no clear explanation as to what use cases this feature will resolve. Until such a time as this is provided, I'm going to mark this as closed.