Re: [Qemu-devel] Should migration with slirp networking (-netdev user) be backward compatible?

[Paolo Bonzini <pbonzini@redhat.com>]

On 13/06/2016 15:39, Guenther Hutzl wrote:
> Hello!
> 
> I have a problem with migration from qemu-v2.6.0 to qemu-v2.5.0. It looks like
> the slirp networking (-netdev user) is not backward compatible. This is the
> output I am seeing on the receiving side:
> 
> qemu-system-x86_64: savevm: unsupported version 4 for 'slirp' v3
> qemu-system-x86_64: load of migration failed: Invalid argument
> 
> This is the little test machine I am trying to migrate (using tiny core linux iso):
> 
> qemu-system-x86_64-2.6.0 -machine pc-i440fx-2.5 \
> -m 512 \
> -cdrom core-current.iso \
> -boot d \
> -nographic \
> -curses \
> -netdev user,id=mynet0 \
> -device virtio-net,netdev=mynet0,id=net0
> 
> The target machine looks like this:
> 
> qemu-system-x86_64-2.5.0 -machine pc-i440fx-2.5 \
> -m 512 \
> -cdrom core-current.iso \
> -boot d \
> -nographic \
> -curses \
> -netdev user,id=mynet0 \
> -device virtio-net,netdev=mynet0,id=net0 \
> -incoming tcp:0:4444
> 
> I am selecting a v2.5-compatible machine so migration to qemu-2.5 should be
> possible. The question is: Is the migration of slirp networking backward
> compatible or is this unsupported for some reason?

Selecting an "old" machine type only ensures that the same hardware is
visible to the guest as in old QEMU, i.e. that your guest will not break
due to buggy drivers or anything like that.  It is also necessary  for
new->old migration, but it's not sufficient.  In general we _try_ to
make migration backwards-compatible, but it's one of the most common
sources of headaches and bugs so it is sometimes broken on purpose (and
with a decent error message).

In this case, SLIRP's migration format was broken because support was
added for IPv6.  New QEMU knows how to load from old QEMU, but not the
other way round.

Paolo