Re: [Qemu-devel] migrating instances from qemu-kvm to qemu

[Paolo Bonzini]

Il 19/03/2014 16:42, Serge E. Hallyn ha scritto:
> Hi,
>
> at https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/1291321
> it was found that migrating running vms from a machine with
> qemu-kvm to one with qemu, migration fails due to some mismatches.
> The first one we usually hit is
>
> Length mismatch: vga.vram: 1000000 in != 800000
>
> while the second one is network card (and I have not gone beyond
> that).

This is just a different default.  Make sure you specify the right model 
on the command line.

> The vga one can be handled on the command line by
> specifying the -global cirrus-vga.vrammem_mb=8.  However that
> doesn't help with a libvirt migration.

You can patch Ubuntu's QEMU to detect old machine types (pc-1.2 and 
earlier) and give cirrus-vga 16MB memory by default.  Migration only 
works with versioned machine types (or between same-version QEMU), so 
it's okay to only do it there.

> I guess this happens at ram_load() unfortunately - is there any
> good way that this could be detected at incoming migration time
> and the virtual hardware modified as needed for migration to
> continue?

It cannot, but the destination can be patched to operate correctly for 
the old machine types, on the assumption that migration from the old 
machine types is always from Ubuntu's qemu-kvm.

Paolo