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

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Nov 24, 2009 at 03:21:34PM +0100, Juan Quintela wrote:
> "Michael S. Tsirkin" <mst@redhat.com> 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<dlaor@redhat.com>  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.

Same device should be able to use many format versions.

As I said in a separate thread, you might want to do migration between
qemu-1.0 to qemu 2.0 to qemu-3.0
and the second part should use a new format, while
all of them must use the same machine.

Doing it this way will let us get rid of some old
formats eventually, without forcing reboots for users.

-- 
MST