Re: [Qemu-devel] [RFC PATCH 0/4] Fix subsection ambiguity in the migration format

[Anthony Liguori]

On 07/29/2011 09:03 AM, Kevin Wolf wrote:
> Am 26.07.2011 14:37, schrieb Anthony Liguori:
> > Hrm, I'm not sure I agree with these conclusions.
> >
> > Management tools should do their best job to create two compatible
> > device models.
> >
> > Given two compatible device models, there *may* be differences in the
> > structure of the device models since we evolve things over time.  We may
> > rename a field, change the type, etc.  To support this, we can use
> > filters both on the destination and receive end to do our best to
> > massage the device model into something compatible.
> >
> > But creating two creating compatible device models is not the job of the
> > migration protocol.  It's the job of management tools.
>
> I'm not sure if I agree with this.
>
> Let's forget about management tools for a moment, and just think of a
> qemu instance with a given set of command line option describing its
> devices. Then you start another instance with different options and
> -incoming and start a migration. The result will be something, but
> definitely not a successfully migrated VM (even though it might look
> like one at first).
>
> This is why I believe that the information about which devices to use
> actually belongs into the migration data. There's no way to make use of
> it with different options.

I agree with you actually.

Right now, it's the management tools job.  The complexity is daunting. 
Recreating the same object model, particularly after hotplug, is 
difficult and in many cases impossible.

I think the primary problem is that there are so many ways to create 
things.  -M pc creates a bunch of stuff that there's no way to create 
individually.  The stuff it creates can sort of be manipulated by using 
-global but not on a per device basis.  Much of it isn't even addressable.

Creating backends is a totally different mechanism and each backend has 
different mechanisms to enumerate and create.

The result is that introspecting what's there and recreating it is 
insanely complex today.

That's the motivation behind QOM.  plug_list lists *everything*.  All 
objects, whether they are created as part of the PIIX3 or whether it's a 
backing file, can be directly addressed and manipulated.

If you look at qsh, there's an import command.  The export command is 
trivial and I don't remember if I've already added it.  But the point is 
that you should be able to 'qsh export' everything and then 'qsh import' 
everything to create the exact same device model in another QEMU instance.

And yeah, this should end up becoming part of the migration protocol.

> > > > 5) Once we're here, we can implement the next 5-year format.  That
> > > > could be ASN.1 and be bidirectional or whatever makes the most sense.
> > > > We could support 50 formats if we wanted to.  As long as the transport
> > > > is distinct from the serialization and compat routines, it really
> > > > doesn't matter.
> > >
> > > This means finishing the VMState support, once there, only thing needs
> > > to change is "copy" the savevm, and change the "visitors" to whatever
> > > else that we need/want.
> >
> > There's no need to "finish" VMState to convert to visitors.  It's just
> > sed -e 's:qemu_put_be32:visit_type_int32:g'
>
> Actually I think the real question is whether we want to have VMState or
> not.

VMState doesn't give me what I want by itself.

I want to be able to marshal the device tree to an in-memory 
representation that can be manipulated.  One approach to doing that is 
first completing VMState, and then writing something that can walk the 
VMState descriptions.  The VMState descriptions are fairly complicated 
but it's doable.

Another approach, which I'm arguing is much simpler, the imperative 
nature of our current serialization and use visitors.

There may be other advantages of a declarative description of VMState 
that would justify completing the conversions.  But I don't think we 
need it to start improving the migration protocol.

Regards,

Anthony Liguori