[libvirt] ARM KVM GICv3 Support

[Christoffer Dall]

Hi all,

I'm trying to figure out what the correct solution for libvirt support
for ARM KVM guests with GICv3 is.

The challenge is that QEMU's virt machine model defaults to GICv2 unless
you specify an additional "-machine gic-version=host" parameter to QEMU,
in which case your VM gets the same GIC version as your host.

Now, I'm told that this is inconvenient for libvirt, because it
typically does not pass any -machine options, is that correct?

Further, there are two additional complications: First, some guest
images may not include GICv3 support.  For Linux, this is relatively old
kernels (prior to v3.17), but other guests may only support GICv2.

Second, some hardware platforms only support GICv2 guests, some only
support GICv3 guests, and some support both GICv2 and GICv3 guests
(those hosts with a GICv3 with the backwards compatibility support).

It is unclear to me how libvirt users will relate to these constraints.
For example, in an OpenStack deployment, will it somehow be easy to tell
which nodes support which GIC version such that administrators can
choose compatible VM images, or how does that work?

The end goal here is to determine if we need to add any functionality to
QEMU to better support libvirt, and what kind of features/glue need to
be added to libvirt for this to work.

Much thanks for any feedback!

-Christoffer