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

[Juan Quintela]

"Michael S. Tsirkin" <address@hidden> wrote:
> On Tue, Nov 24, 2009 at 12:39:50PM +0200, Dor Laor wrote:
>> On 11/23/2009 02:15 PM, Juan Quintela wrote:
>>> Dor Laor<address@hidden>  wrote:
>>>> >  In the last couple of days we discovered some issues regarding stable
>>>> >  ABI and the robustness of the live migration protocol. Let's just jump
>>>> >  right into it, ordered by complexity:
>>>> >
>>>> >  1. Control*every*  feature exposed to the guest by qemu cmdline:
>>>> >
>>>> >      While thinking on cross version migration, and reviewing some
>>>> >      patches, I noticed that there are many times that we use feature 
>>>> > bits
>>>> >      in order to expose functionality for the guest driver - example:
>>>> >      VIRTIO_BLK_F_BARRIER, but we do not control it from qemu cmdline.
>>> In my opinion this is madness, qemu command line is already too
>>> complicated.  I agree with anthony to put it in the command line.
>>
>> Qemu's cmdline is currently our config file.. Actually there is nothing  
>> wrong with it. Human users shouldn't be interested with these changes  
>> and management software should not have problem manipulating it.
>> We do need flexibility of controlling our features like any other  
>> software component.
>>
>>> I will go further, and think that this kind of issues should be put into
>>> the machine type.
>>>
>>> If you start qemu with -M pc-0.10, it should save the state in a 0.10
>>> compatible way (that don't happens at the moment, but it should work
>>> that way).
>>
>> That's the idea - to keep it part of qdev and by default use it with -M.
>
> I think we want to keep these things separate:
> machine description should be for things that
> are both guest visible and not changeable by guest,
> so it absolutely must stay constant as long as guest
> it alive.

That is exactly what we need here, that version of the savevm protocol
for each device is the same.

Later, Juan.