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

[Cole Robinson]

On 03/21/2014 01:00 PM, Serge E. Hallyn wrote:
> Quoting Cole Robinson (address@hidden):
>> On 03/19/2014 12:16 PM, Serge E. Hallyn wrote:
>>> Quoting Serge E. Hallyn (address@hidden):
>>>> Quoting Paolo Bonzini (address@hidden):
>>>>> 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.

- Cole