Re: [PATCH v3] hw/pci/pci_bridge: ensure PCIe slots have only one slot

[Roman Kagan]

On Wed, Jul 20, 2022 at 11:44:26AM +0100, Daniel P. Berrangé wrote:
> On Wed, Jul 20, 2022 at 01:25:55PM +0300, Roman Kagan wrote:
> > It's possible to create non-working configurations by attaching a device
> > to a derivative of PCIe slot (pcie-root-port, ioh3420, etc) and
> > specifying a slot number other that zero, e.g.:
> > 
> >     -device pcie-root-port,id=s0,... \
> >     -device virtio-blk-pci,bus=s0,addr=4,...
> > 
> > Make QEMU reject such configurations and only allow addr=0 on the
> > secondary bus of a PCIe slot.
> 
> What do you mean by 'non-working' in this case.  The guest OS boots
> OK, but I indeed don't see the device in the guest, but IIUC it was
> said that was just because Linux doesn't scan for a non-zero slot.

Right.  I don't remember if it was Linux or firmware or both but indeed
at least Linux guests don't see devices if attached to a PCIe slot at
addr != 0.  (Which is kinda natural for a thing called "slot", isn't it?)

> That wouldn't be a broken config from QEMU's POV though, merely a
> guest OS limitation ?

Strictly speaking it wouldn't, indeed.  But we've had created such a
configuration (due to a bug in our management layer) and spent
non-negligible time trying to figure out why the attached device didn't
appear in the guest.  So I thought it made sense to reject a
configuration which is known to confuse guests.  Doesn't it?

Thanks,
Roman.