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[Xen-devel] Re: A proposal - binary

On Fri, 4 Aug 2006, Rusty Russell wrote:

On Thu, 2006-08-03 at 22:53 -0700, Andrew Morton wrote:
On Fri, 04 Aug 2006 15:04:35 +1000
Rusty Russell <rusty@xxxxxxxxxxxxxxx> wrote:

On Thu, 2006-08-03 at 21:18 -0700, Andrew Morton wrote:
Everywhere in the kernel where we have multiple implementations we want
to select at runtime, we use an ops struct.  Why should the choice of
Xen/VMI/native/other be any different?

VMI is being proposed as an appropriate way to connect Linux to Xen.  If
that is true then no other glue is needed.

Sorry, this is wrong.  VMI was proposed as the appropriate way to
connect Linux to Xen, *and* native, *and* VMWare's hypervisors (and
others).  This way one Linux binary can boot on all three, using
different VMI blobs.

Yes, we could force native and Xen to work via VMI, but the result would
be less clear, less maintainable, and gratuitously different from
elsewhere in the kernel.

I suspect others would disagree with that.  We're at the stage of needing
to see code to settle this.

Wrong again.  We've *seen* the code for VMI, and fairly hairy.  Seeing
the native-implementation and Xen-implementation VMI blobs will not make
it less hairy!

 And, of course, unlike paravirt_ops where we
can change and add ops at any time, we can't similarly change the VMI
interface because it's an ABI (that's the point: the hypervisor can
provide the implementation).

hm.  Dunno.  ABIs can be uprevved.  Perhaps.

Certainly VMI can be.  But I'd prefer to leave the excellent hackers at
VMWare with the task of maintaining their ABI, and let Linux hackers
(most of whom will run native) manipulate paravirt_ops freely.

We're not good at maintaining ABIs.  We're going to be especially bad at
maintaining an ABI when the 99% of us running native will never notice
the breakage.

some questions from a user. pleas point out where I am misunderstanding things.

one of the big uses of virtualization will be to run things in sandboxes, when people do this they typicaly migrate the sandbox from system to system over time (working with chroot sandboxes I've seen some HUGE skews between what's running in the sandbox and what's running in the host). If the interface between the guest kernel and the hypervisor isn't fixed how could somone run a 2.6.19 guest and a 2.6.30 guest at the same time?

if it's only a source-level API this implies that when you move your host kernel from 2.6.19 to 2.6.25 you would need to recompile your 2.6.19 guest kernel to support the modifications. where are the patches going to come from to do this?

It seems to me from reading this thread that the PowerPC and S390 have a ABI defined, specificly defined by the hardware in the case of PowerPC and by the externaly maintained, Linux-independant hypervisor (which is effectivly the hardware) in the case of the s390.

If there's going to be long-term compatability between different hosts and guests there need some limits to what can change.

needing to uprev the host when you uprev a guest is acceptable

needing to uprev a guest when you uprev a host is not.

this basicly boils down to 'once you expose an interface to a user it can't change', with the interface that's being exposed being the calls that the guest makes to the host.

David Lang

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