Alex Williamson <alex.williamson@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