I'd like to have one KVM kernel image which is suitable for the real
hardware host and the virtio based guest. The single zImage plus
Device Tree work seem like a great way to do this.

We're currently using the vexpress-a15 on a Fast Model as the host and
a vexpress-a15 as the guest. Device Tree support is required to
describe the virtio-mmio devices. As a bonus, the vexpress-a9 and
vexpress-a15 are the same hardware with a different memory map and can
help demonstrate the Device Tree support.

What are the plans for single zImage? Where does the vexpress-a15 fit
in with that? Could I bump it to the front of the list?

single zImage is already used for describing our final goal of having a single zImage for all boards... I think there is some way to go for that (especially since we do not yet have a single source tree). For stuff that can be tweaked through DT right now, I don't see why we couldn't have a single zImage ... e.g. like in your case having ability to boot vexpress-a15 in two different setups through different device tree...

Most likely would require some platform plumming to ship two or more DTs for one kernel.

What do we need for that? I guess we need a way to have two different device trees produced into the same image/hwpack and make it easy to decide at deploy time what u-boot is supposed to select?

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