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

[Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>]

On 20/07/2022 12:00, Roman Kagan wrote:

> 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?

This does seem a bit odd. What does the output of "info qtree" look like for your 
non-working configuration?


ATB,

Mark.