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,

That's wrong I think. Anything attached to RC is an integrated
endpoint, and these can be PCI devices.

> 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).

IMO, we really need to grow an interface to query this kind of thing.

-- 
MST