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

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

Quoting Cole Robinson (crobinso@redhat.com):
> On 03/21/2014 01:00 PM, Serge E. Hallyn wrote:
> > 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?
> > 
> 
> Line 149 of the patch changes the cirrus-vga default memory for old machine
> types, seems to fix things in my testing.

Hh, hm, how did i miss that.

thanks,
-serge