Re: [Qemu-devel] [PATCH] Change virtio-console to PCI_CLASS_SERIAL_OTHER

[Dor Laor <dlaor@redhat.com>]

Anthony Liguori wrote:
> Paul Brook wrote:
>   
>>>>>   - the device model cannot change or the guest OS will get confused
>>>>>         
>>>>>           
>>>> IMHO think the only sane response is "don't do that". Trying to support
>>>> migration between different qemu versions just isn't worth the pain.
>>>>       
>>>>         
>>> It is very worth the pain.  I consider it a core requirement.
>>>     
>>>       
>> We disagree then. You're effectively requiring bug-compatibility.
>>   
>>     
>
> You can take it to an extreme and require bug-compatibility, but I don't
> think that's necessary.  I think there's room for being reasonable.
>
>   
>> This may be reasonable for a stable branch, but is not something I have any 
>> interest in across different release cycles. IMHO major VM upgrades should be 
>> considered the same as real hardware or firmware upgrades.
>>   
>>     
>
> I think major VM upgrades is something that can be considered
> occasionally but every 6 months would be pretty painful to most users. 
> I think we ought to make a best effort to maintain compatibility with
> older versions as long as it's reasonable.
>
>   
Supporting live migration between different versions of qemu is a 
stretch goal. In the past
2 month we fixed about 10 basic live migration issues and mainline keeps 
breaking ;(

Nevertheless, as Anthony states, guest ABI should be rock stable.
We should have mechanism (machine conf format++) that will enable us
to configure pci addresses, cpuid entries, memory layout, device 
existence (hpet, virtio-console,..), etc.

Anyhow, I did sent a patch to allow virtio block to parametrized class 
value -
http://lists.gnu.org/archive/html/qemu-devel/2009-05/msg01189.html
There wasn't a perfect match with finding the right interface to change 
it, happy to hear
other alternatives if you're not happy with it.

Thanks,
Dor

> Regards,
>
> Anthony Liguori
>
>   
>> Paul
>>   
>>     
>
>
>
>