[libvirt] How should libvirt apps enable virtio-pci for aarch64?

[Cole Robinson]

Hi all,

I'm trying to figure out how apps should request virtio-pci for libvirt + qemu
+ arm/aarch64. Let me provide some background.

qemu's arm/aarch64 original virtio support is via virtio-mmio, libvirt XML
<address type='virtio-mmio'/>. Currently this is what libvirt sets as the
address default for all arm/aarch64 virtio devices in the XML. Long term
though all arm virt will likely be using virtio-pci: it's faster, enables
hotplug, is more x86 like, etc.

Support for virtio-pci is newer and not as widespread. qemu has had the
necessary support since 2.4 at least, but the guest side isn't well
distributed yet. For example, Fedora 23 and earlier don't work out of the box
with virtio-pci. Internal RHELSA (RHEL Server for Aarch64) builds have it
recently working AFAIK.

Libvirt has some support for enabling virtio-pci with aarch64, commits added
by Pavel Fedin in v1.2.19. (See e8d55172544c1fafe31a9e09346bdebca4f0d6f9). The
patches add a PCIe controller automatically to the XML (and qemu commandline)
if qemu-system-aarch64 supports it. However virtio-mmio is still used as the
default virtio address, given the current lack of OS support.

So we are at the point where libvirt apps want to enable this, but presently
there isn't a good solution; the only option is to fully allocate <address
type='pci' ...> for each virtio device in the XML. This is suboptimal for 2
reasons:

#1) apps need to duplicate libvirt's non-trivial address type=pci allocation logic

#2) apps have to add an <address> block for every virtio device, which is less
friendly than the x86 case where this is rarely required. Any XML device
snippets that work for x86 likely won't give the desired result for aarch64,
since they will default to virtio-mmio. Think virsh attach-device/attach-disk
commands


Here are some possible solutions:

* Drop the current behavior of adding a PCIe controller unconditionally, and
instead require apps to specify it in the XML. Then, if libvirt sees a PCIe
controller in the XML, default the virtio address type to pci. Apps will know
if the OS they are installing supports virtio-pci (eventually via libosinfo),
so this is the way we can implicitly ask libvirt 'allocate us pci addresses'

Upsides:
- Solves both the stated problems.
- Simplest addition for applications IMO

Downsides:
- Requires a libvirt behavior change, no longer adding the PCIe controller by
default. But in practice I don't think it will really affect anyone, since
there isn't really any OS support for virtio-pci yet, and no apps support it
either AFAIK.
- The PCIe controller is not strictly about virtio-pci, it's for enabling
plain emulated PCI devices as well. So there is a use case for using the PCIe
controller for a graphics card even while your OS doesn't yet support
virtio-pci. In the big picture though this is a small time window with current
OS, and users can work around it by manually requesting <address
type='virtio-mmio'/>, so medium/long term this isn't a big deal IMO
- The PCIe controller XML is:
    <controller type='pci' index='0' model='pcie-root'/>
    <controller type='pci' index='1' model='dmi-to-pci-bridge'/>
    <controller type='pci' index='2' model='pci-bridge'/>
I have no idea if that's always going to be the expected XML, maybe it's not
wise to hardcode that in apps. Laine?


* Next idea: Users specify something like like <address type='pci'/> and
libvirt fills in the address for us.

Upsides:
- We can stick with the current PCIe controller default and avoid some of the
problems mentioned above.
- An auto address feature may be useful in other contexts as well.

Downsides:
- Seems potentially tricky to implement in libvirt code. There's many places
that check type=pci and key off that, seems like it would be easy to miss
updating a check and cause regressions. Maybe we could add a new type like
auto-pci to make it explicit. There's probably some implementation trick to
make this safe, but at first glance it looked a little dicey.
- Doesn't really solve problem #2 mentioned up above... maybe we could change
the address allocation logic to default to virtio-pci if there's already a
virtio-pci device in the XML. But it's more work.
- More work for apps, but nothing horrible.


* Change the default address type from virtio-mmio to pci, if qemu supports
it. I'm listing this for completeness. In the short term this doesn't make
sense as there isn't any OS releases that will work with this default. However
it might be worth considering for the future, maybe keying off a particular
qemu version or machine type. I suspect 2 years from now no one is going to be
using virtio-mmio so long term it's not an ideal default.


I think the first option is best (keying off the PCIe controller specified by
the user), with a longer term plan to change the default from mmio to pci. But
I'm not really sold on anything either way. So I'm interested if anyone else
has ideas.

Thanks,
Cole