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If they are two packages, and Linux breaks the interface between kernel and hypervisor, then the fix would be to redesign the Linux change to cope with keeping the same ABI to the hypervisor. This is a burden that the maintainers do not want to carry.
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That's an understandable concern. But Keir Fraser has unequivocally stated that he does not expect that. It's Xen's job to be backwards compatible with older kernels if it wants to be. It's not Linux's job to be backwards compatible with older hypervisors. If the ABI changes in Linux, Xen will upgrade to match; and if someone upgrades Linux over a dom0<->xen ABI change, then they will have to upgrade Xen over that same ABI change as well.