Re: [Qemu-devel] Attaching PCI devices to the PCIe root complex

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Sep 24, 2013 at 06:01:02AM -0400, Laine Stump wrote:
> When I added support for the Q35-based machinetypes to libvirt, I
> specifically prohibited attaching any PCI devices (with the exception of
> graphics controllers) to the PCIe root complex, and had planned to
> prevent attaching them to PCIe root ports (ioh3420 device) and PCIe
> downstream switch ports (xio-3130 device) as well. I did this because,
> even though qemu currently allows attaching a normal PCI device in any
> of these three places, the restriction exists for real hardware and I
> didn't see any guarantee that qemu wouldn't add the restriction in the
> future in order to more closely emulate real hardware.
> 
> However, since I did that, I've learned that many of the qemu "pci"
> devices really should be considered as "pci or pcie". Gerd Hoffman lists
> some of these cases in a bug he filed against libvirt:
> 
>    https://bugzilla.redhat.com/show_bug.cgi?id=1003983
> 
> I would like to loosen up the restrictions in libvirt, but want to make
> sure that I don't allow something that could later be forbidden by qemu
> (thus creating a compatibility problem during upgrades). Beyond Gerd's
> specific requests to allow ehci, uhci, and hda controllers to attach to
> PCIe ports, are there any other devices that I specifically should or
> shouldn't allow? (I would rather be conservative in what I allow - it's
> easy to allow more things later, but nearly impossible to revoke
> permission once it's been allowed).


Thinking some more about it.
At the moment qemu is very flexible, allowing you
to create all kind of illegal configurations.
How about we teach qemu to reject illegal configs,
and libvirt simply tries to create them and finds out
what's legal this way?

Same would work for detecting hotplug slots, etc.