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

[Mark Cave-Ayland]

On 21/07/2022 15:28, Roman Kagan wrote:

(lots cut)

> In the guest (Fedora 34):
>
> [root@test ~]# lspci -tv
> -[0000:00]-+-00.0  Intel Corporation 82G33/G31/P35/P31 Express DRAM Controller
>             +-01.0  Device 1234:1111
>             +-02.0  Red Hat, Inc. QEMU XHCI Host Controller
>             +-05.0-[01]----00.0  Red Hat, Inc. Virtio block device
>             +-05.1-[02]----00.0  Red Hat, Inc. Virtio network device
>             +-05.2-[03]--
>             +-05.3-[04]--
>             +-1f.0  Intel Corporation 82801IB (ICH9) LPC Interface Controller
>             \-1f.3  Intel Corporation 82801I (ICH9 Family) SMBus Controller
>
> Changing addr of the second disk from 4 to 0 makes it appear in the
> guest.
>
> What exactly do you find odd?

Thanks for this, the part I wasn't sure about was whether the device ids in the 
command line matched the primary PCI bus or the secondary PCI bus.

In that case I suspect that the enumeration of non-zero PCIe devices fails in Linux 
because of the logic here: 
https://github.com/torvalds/linux/blob/master/drivers/pci/probe.c#L2622.

I don't have a copy of the PCIe specification, but assuming the comment is true then 
your patch looks correct to me. I think it would be worth adding a similar comment 
and reference to your patch to explain why the logic is required, which should also 
help the PCI maintainers during review.


ATB,

Mark.