Re: Merge Xen (the hypervisor) into Linux

Gerd Hoffmann
2009-06-03T11:43:42Z

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.

cheers,
Gerd

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