Christoph Lameter wrote:

And it seems that the hooks are not generic but bound to a particular 
hypervisor. Should the Xen specific stuff not be in the binary blob?

Xen has no "binary blob".  It needs guests to cooperate with it by
making hypercalls; all that code is in the Xen implementation of the
paravirt_ops interface, which is just ordinary code which lives in the
kernel sources.

    J