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

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

Quoting Cole Robinson (crobinso@redhat.com):
> On 03/19/2014 12:16 PM, Serge E. Hallyn wrote:
> > 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.
> > 
> 
> FWIW, we've carried a patch like this in Fedora 18, 19, and 20:
> 
> http://pkgs.fedoraproject.org/cgit/qemu.git/tree/0001-Fix-migration-from-qemu-kvm.patch?h=f20
> 
> As you say, it breaks incoming migration from stock qemu.git. But we never
> shipped qemu.git prior to F18 (well, not in any timeframe that matters here),
> so back compat wasn't a real concern.
> 
> For Fedora 21 we are dropping this patch, since there's been enough releases
> for the people to transition:
> 
> http://blog.wikichoon.com/2013/12/kvm-migration-from-fedora-17-to-fedora.html

Thanks, Cole.  I notice it doesn't do anything for -vga cirrus - is that
just bc in general fedora users tend to use -vga qxl?

-serge