Re: [Qemu-devel] Re: Live migration protocol, device features, ABIs and other beasts

[Michael S. Tsirkin]

On Wed, Nov 25, 2009 at 02:36:49PM +0100, Gerd Hoffmann wrote:
> On 11/24/09 15:05, Michael S. Tsirkin wrote:
>> On Mon, Nov 23, 2009 at 03:13:59PM +0100, Juan Quintela wrote:
>>>> But this would only kick in when using pc-0.11 or something, right?
>>>
>>> Yeap.
>>>
>>> At this point, pc-0.10 is just:
>>>
>>> static QEMUMachine pc_machine_v0_10 = {
>>>      .name = "pc-0.10",
>>>      .desc = "Standard PC, qemu 0.10",
>>>      .init = pc_init_pci,
>>>      .max_cpus = 255,
>>>      .compat_props = (CompatProperty[]) {
>>>          {
>>>              .driver   = "virtio-blk-pci",
>>>              .property = "class",
>>>              .value    = stringify(PCI_CLASS_STORAGE_OTHER),
>>>          },{
>>>              .driver   = "virtio-console-pci",
>>>              .property = "class",
>>>              .value    = stringify(PCI_CLASS_DISPLAY_OTHER),
>>>          },{
>>>              .driver   = "virtio-net-pci",
>>>              .property = "vectors",
>>>              .value    = stringify(0),
>>>          },{
>>>              .driver   = "virtio-blk-pci",
>>>              .property = "vectors",
>>>              .value    = stringify(0),
>>>          },
>>>          { /* end of list */ }
>>>      },
>>>
>>> But to really make it work, we need to take a list of each savevm format
>>> change and put it here.  Notice that several changes are needed:
>>> - savevm infrastructure save functions don't know about version id
>>> - devices don't know to "behave" as other version
>>> - other things that I have probably missed
>>>
>>> Later, Juan.
>>
>> Why do you think this the right place for it, I wonder?
>> This describes the machine, it does not seem to have
>> anything to do with how we migrate it.
>
> Well.  It turns off MSI for virtio-net-pci when you start it with -M  
> pc-0.10.  Which makes virtio-net-pci savevm sections compatible with the  
> qemu 0.10 ...
>
> We could add a DeviceState->savevm field and make that available as  
> property for devices which need to support multiple versions.  Then you  
> we can use the compat properties to switch back to the older format with  
> -M pc-0.10.
>
> cheers,
>   Gerd

I'm confused sorry. Of course when you want to migrate to qemu 0.10
you must have a compatible machine. And savevm format has nothing
to do with it IMO, so MSI is orthogonal to this discussion.
It just shows that it was smart not to save MSI state when
MSI is not present (/me pats self on the back).

In this thread we were discussing changes like pvclock bug,
where we change savevm format without changing the machine,
or almost without changing the machine.

-- 
MST