Re: [Qemu-devel] [PATCH] Change virtio-console to PCI_CLASS_SERIAL_OTHER

[Jamie Lokier <jamie@shareable.org>]

Anthony Liguori wrote:
> The problem is that the device model a guest sees is an ABI.  If we
> change the ABI between versions, the guest make break.  What I'm
> proposing is that while we can change the default guest ABI between
> versions, we must provide a way to recreate older forms of the guest ABI.
> 
> A good example of guests breaking is Windows activation failing.  It has
> happened more than once that changes in QEMU cause Windows guests to
> force reactivation.  In more extreme instances, a guest may break that
> used to work before.

I agree.

As far as I'm concerned, a major use of virtualisation is to be able
to run old guests on newer hosts without having to keep changing the
guest OS, including reinstalling its drivers etc.

Because of this guest ABI compatibility is perhaps more important than
it would be for othe rapplications.

Windows activation is particularly ugly, but maybe we can special case
that by making known relevant serial numbers, asset tags
etc. configurable in the device model?

You can avoid problems by sticking with a fixed QEMU/KVM version of
course, but then you don't get the benefits of development like
performance improvements and bug fixes.  And in the long run, it
doesn't work because older QEMU/KVMs don't work properly (or at all)
on later host systems.

If newer Qemu's can't do that, we'll eventually be in the curious
position of having to use nested virtualisation - to run an older Qemu
on an older nested 'host'.  Sounds annoying :-)

As soon as the device model config file is used, I expect all this
will get easier to manage than it is now.  The main thing then will be
to ensure there are options to recreate old guest modes for particular
devices.  There's probably just a few such options, so not much of a
maintenance burden.

-- Jamie