I'm not sure we can create something which works on both kvm and xen.
The memory management model is quite different ...
On xen the hypervisor manages all memory. Guests can allow other guests
to access specific pages (using grant tables). In theory any guest <=>
guest communication is possible. In practice is mostly guest <=> dom0
because guests access their virtual hardware that way. dom0 is the
priviledged guest which owns any hardware not managed by xen itself.
Xen guests can ask the hypervisor to update the mapping of guest
physical pages. They can ballon down (unmap and free pages). They can
ballon up (ask the hypervisor to map fresh pages). They can map pages
exported by other guests using grant tables. xen-zcopy makes heavy use
of this. It balloons down, to make room in the guest physical address
space, then goes map the exported pages there, finally composes a
dma-buf.
On kvm qemu manages all guest memory. qemu also has all guest memory
mapped, so a grant-table like mechanism isn't needed to implement
virtual devices. qemu can decide how it backs memory for the guest.
qemu propagates the guest memory map to the kvm driver in the linux
kernel. kvm guests have some control over the guest memory map, for
example they can map pci bars wherever they want in their guest physical
address space by programming the base registers accordingly, but unlike
xen guests they can't ask the host to remap individual pages.
Due to qemu having all guest memory mapped virtual devices are typically
designed to have the guest allocate resources, then notify the host
where they are located. This is where the udmabuf idea comes from:
Guest tells the host (qemu) where the gem object is, and qemu then can
create a dmabuf backed by those pages to pass it on to other processes
such as the wayland display server. Possibly even without the guest
explicitly asking for it, i.e. export the framebuffer placed by the
guest in the (virtual) vga pci memory bar as dma-buf. And I can imagine
that this is useful outsize virtualization too.
I fail to see any common ground for xen-zcopy and udmabuf ...