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

From: Laine Stump
Subject: [Qemu-devel] Attaching PCI devices to the PCIe root complex
Date: Tue, 24 Sep 2013 06:01:02 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130625 Thunderbird/17.0.7

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:


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).

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