Re: [libvirt] [Qemu-devel] q35 machine type and libvirt.
Alex Williamson <alex.williamson(a)redhat.com> writes:
On Wed, 2013-02-27 at 13:20 -0500, Laine Stump wrote:
> On 02/06/2013 02:13 PM, Laine Stump wrote:
> > Now that qemu is getting the q35 machine type, libvirt needs to support it.
>
> In an attempt to make sure that libvirt actually does something useful
> with qemu's new machine type, I'm revisiting this topic and trying to
> get a more thorough understanding.
>
> I've developed my own list of what I *think* are the reasons for wanting
> a new machine type in qemu (and from that it should be more apparent
> what libvirt needs to do with it), and am wondering how far off the mark
> I am:
>
Right, somehow libvirt needs to know or qemu needs to tell it something
about the devices it's plugging in. If you were to grab your trust
10/100Mbps Legacy PCI ethernet card and try to plug it into a
motherboard you'd quickly notice that you can only plug it into certain
slots. This is the same problem. PCI device are attached to a legacy
PCI bus, which means it needs to be behind a PCIe-to-PCI bridge. Legacy
Endpoints and Endpoints are plugin PCIe cards, so they need to be
plugged in behind a PCIe switch or root port. Integrated Endpoints are
motherboard components, per the spec, they shouldn't even really be
hotplug-able. They attach directly to the root complex.
We could do this with QOM. The chipset could have a set of link
properties for the integrated devices. For instance:
Q35Chipset
Link<E1000> integrated_nic;
Link<StdVGA> integrated_vga;
...
We should prevent PCI bus plugging for slots "owned" by integrated
devices. libvirt has a way to probe for links that it can add including
what types are allowed for it.
Regards,
Anthony Liguori
> * support some new emulated chipset devices?
-M q35 + -device ioh3420 (root port) + -device i82801b11-bridge
(pcie-to-pci bridge)
> * Anything else specific required to passthrough pcie devices?
I just sent out an RFC asking about VGA routing, it's possible libvirt
will need to understand some concept of a primary VGA device and specify
it somehow to QEMU.
It's also possible (probable) that we'll make assigned devices able to
mangle PCIe types to make it more flexible where they can be attached
(for instance you may not want to have your HD audio device exposed as a
root complex device if that disables hotplug, so we may mangle it to
look like a regular endpoint. Windows is picky about root complex
devices, so we may also mangle endpoint to integrated endpoint as well).
Also need to be aware that all PCIe "buses" except the root complex are
actually point-to-point links, so a root port connects to one PCIe
device (which may be multifunction). PCIe switches are needed to get
fan out to connect multiple devices, one device per downstream port. It
would be interesting how much we can abuse the topology in a virtual
system, but it's probably best not to confuse guests. Thanks,
Alex