On Mon, Mar 20, 2006 at 08:37:15PM -0500, Jed Davis wrote: > Geert Hendrickx <ghen%telenet.be@localhost> writes: > > > How much is Xen backward/forward compatible? e.g. once there is support > > for NetBSD as dom0 on Xen 3, can I run a NetBSD 3.0 domU (i.e. for Xen 2) > > with it? > > My understanding is that it's not compatible at all, as important > interfaces (domain control interface vs. XenBus, introduction of grant > tables) have been completely changed. Likewise, that's my understanding: it basically represents ports to two different "machines". There is theoretical caveat, mostly as an aside. Xen3 also includes a shim that runs within the current framework as a domU, and provides additional emulation on top of that interface to look like a bare-metal machine, for native operating systems to run in, like windows. This uses bits of qemu for all of the things not in the base hypervisor, like video bios and/boot cpu startup, and relies on the VT extensions in newer processors as well. I'm looking forward to this. The point, however, is that it may also be feasible/possible to write a similar shim for back compatibility, that runs as a xen3 domU and presents a xen2 hypervisor interface to older xen kernels, translating as necessary. Thus the core hypervisor doesn't need to know about maintaining compatibility. I recall discussion of this possibility at one point, with the conclusion that if someone wants to write it they can go ahead. I don't know (and rather doubt) that anyone ever has. -- Dan.
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