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

["Serge E. Hallyn" <serge@hallyn.com>]

Quoting Serge E. Hallyn (serge@hallyn.com):
> Quoting Paolo Bonzini (pbonzini@redhat.com):
> > 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
> 
> Ah, thank you, that assumption should be safe and sounds like a
> great idea.

Although, some people are using newer qemu with '-M pc-1.0'.  So we'd be
stopping thing from migrating to support the people coming from qemu-kvm.
Hmm.

-serge