Re: [libvirt] [PATCH 2/2] HACK: qemu: aarch64: Use virtio-pci if user specifies PCI controller

On 01/28/2016 04:14 PM, Cole Robinson wrote: As you've noted below, this isn't correct. aarch64 machvirt has no implicit pci-root controller (aka "pci.0"). It instead has a pcie-root controller ("pcie.0"). Since a pci[e]-root controller cannot be explicitly added, by definition this couldn't work. That's not the right error, but it's caused by the fact that libvirt wants the pci-bridge device to be plugged into a standard PCI slot, but all the slots of pcie-root are PCIe slots. Since we now know that qemu doesn't mind if any standard PCI device is plugged into a PCIe slot, the decision of how we want to solve this problem depends on whether or not we want the devices in question to be hot-pluggable - the ports of pcie-root do not support hot-plugging devices (at least on Q35), while the ports on pci-bridge do. So if we require that all devices be hot-pluggable, then we have a few choices: 1) create the same PCI controller Frankenstein we currently have for Q35 - a dmi-to-pci-bridge plugged into pcie-root, and a pci-bridge plugged into dmi-to-pci-bridge. This is easiest because it already works, but it does create an extra unnecessary controller. 2) auto-add a pci-bridge in cases when there is a pcie-root but not standard PCI slots. This would take only a slight amount more work. 3) auto-add a pcie-root-port to each port of the pcie-root controller. This would still leave us with PCIe ports, so we would need to teach libvirt that it's okay to plug PCI devices into PCIe ports. If we don't require hot-pluggability, then we can just teach the address-assignment code that PCI devices can plug into non-hotpluggable PCIe ports and we're done. Or we can do a hybrid that's kind of a continuation of the "use PCI if it's available, otherwise mmio" - we could do this: A) If there are any standard PCI slots, then auto-assign to PCI slots (creating new pci-bridge controllers s necessary) B) else if there are any PCIe slots, then auto-assign to hot-pluggable PCIe if available, or straight PCIe if not. C) else use virtio-mmio. ------------------------------------------- Mixed in with all of this discussion is my thinking that we should have some way to specify, in XML, constraints for the address of each device *without specifying the address itself*. Things we need to be able to specify: 1) Is a PCI-only vs. PCIe-only vs. either one (maybe this could be used in the future to constrain to virtio-mmio as well)? 2) Must the device be hot-pluggable? (default would be yes) 3) guest-side NUMA node? (I'm not sure if this needs to be user specifiable - in the case of a vfio-assigned device, I think all we need to to inform the guest which NUMA node the device is on in the host (via putting it on a PXB controller that is configured with that same NUMA node number). For emulated devices - is there any use to putting an *emulated* device on the same controller as a particular vfio-assigned device that is on a specific node? If not, then maybe it will never matter). It would be better if these "address constraints" were in a different part of the XML than the <address> element itself - this would maintain the simplicity of being able to just remove all <address> elements in order to force libvirt to re-assign all device addresses. This isn't something that needs doing immediately, but worth keeping in mind while putting together something that works for aarch64. I think that's a dead-end that we would have to back-track on, so probably not a good solution even temporarily. Here's an attempt at a plan: 1) change the PCI address assignment code so that for aarch64/virt it prefers PCIe addresses, but still requires hot-pluggable (currently it almost always prefers PCI, and requires hot-pluggable). (alternate - if aarch64 doesn't support pcie-root-port or pcie-switch-*-port, then don't require hot-pluggable either). 2) put something on the front of that that checks for existence of pcie-root, and if it's not found, uses virtio-mmio instead (is there something already that auto-adds the virtio-mmio address? I haven't looked and am too lazy to do so now). At this point, as long as you manually add a bunch of pcie-root-port controllers along with the manual pcie-root, everything should just work. Then we would go to step 3: 3) enhance the auto-assign code so that, in addition to auto-adding a pci-bridge when needed, it would auto-add either a single pcie-root-port or a pcie-switch-upstream-port and 32 pcie-switch-downstream-ports anytime a hotpluggable PCIe port was needed and couldn't be found. (the latter assumes that aarch64 supports those controllers). Does that make any sense? I could try to code some of this up if you could test it (or help me get setup to test it myself).