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

Laine Stump laine at laine.org
Wed Feb 17 20:03:18 UTC 2016

On 01/28/2016 04:14 PM, Cole Robinson wrote:
> If a user manually specifies this XML snippet for aarch64 machvirt:
>    <controller type='pci' index='0' model='pci-root'/>

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.

> Libvirt will interpret this to mean that the OS supports virtio-pci,
> and will allocate PCI addresses (instead of virtio-mmio) for virtio
> devices.
> This is a giant hack. Trying to improve it led me into the maze of PCI
> address code and I gave up for now. Here are the issues:
> * I'd prefer that to be model='pcie-root' which matches what
> qemu-system-aarch64 -M virt actually provides by default... however
> libvirt isn't happy with a single pcie-root specified by the user, it
> will error with:
> error: unsupported configuration: failed to create PCI bridge on bus 1: too many devices with fixed addresses

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 

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.

> Instead this patch uses hacks to make pci-root use the pcie.0 bus for
> aarch64, since that code path already works.

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).

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