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

[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

- Cole