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[Xen-users] Re: Merge Xen (the hypervisor) into Linux

On 06/03/09 13:15, Theodore Tso wrote:
On Wed, Jun 03, 2009 at 11:09:39AM +0200, Gerd Hoffmann wrote:
It wasn't my intention to imply that.  The interface can be extended
when needed.  PAT support will probably be such a case.  Changing it in
incompatible ways isn't going to work though.

But that means that if there is some fundamentally broken piece of
dom0 design, that the Linux kernel will be stuck with it ***forever***
and it will contaminate code paths and make the code harder to
maintain ***forever*** if we consent to the Xen merge?

No. Xen is stuck with it forever (or at least for a few releases). Even when adding new & better dom0/xen interfaces in the merge process Xen has to keep the old ones to handle the other dom0 guests (NetBSD, Solaris, old 2.6.18 out-of-tree linux kernel). Pretty much like the linux kernel has to keep old syscalls to not break the ABI for the applications, xen has to maintain old hypercalls[1].

Other way around: Apps can use new system calls only when running one recent kernels, and they have to deal with -ENOSYS. Likewise it might be that the pv_ops-based dom0 kernel can provide some features only when running on a recent hypervisor. That will likely be the case for PAT.


[1] and other interfaces like trap'n'emulate certain instructions.

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