Re: [Qemu-devel] Re: new->old version migration

[Michael S. Tsirkin]

On Tue, Feb 08, 2011 at 01:42:59AM -0600, Anthony Liguori wrote:
> On 02/08/2011 12:42 AM, Michael S. Tsirkin wrote:
> >On Mon, Feb 07, 2011 at 08:23:08PM -0600, Anthony Liguori wrote:
> >>On 02/07/2011 03:52 PM, Michael S. Tsirkin wrote:
> >>>How does it? We need to know we are saving in 0.13
> >>>format and skip the new subsection, otherwise
> >>>0.13 will see a subsection it does not recognize
> >>>and exit.
> >>If you used subsections for flow control, presumably you would only
> >>send the new savevm data if you had data buffered.
> >>
> >>If you add a qdev property to enable/disable flow control, then if
> >>it's disabled, you naturally would never send the subsection because
> >>you'd never buffer data.  So no explicit code is needed to support
> >>migration.
> >But the result is we get a new property that we can never remove
> >as any qdev property is part of interface.
> >
> >>The difficult case is when you truly need to change the savevm
> >>version.  I don't think we have a proper fix for this because
> >>versions are linear so the proposed patch certainly wouldn't be a
> >>good way to do it.  if flow_control=0 causes savevm 3 to be used
> >>instead of 4, and then the next_feature=0 causes savevm 4 to be used
> >>instead of 5, the semantics of flow_control=0,next_feature=1 becomes
> >>problematic.
> >>
> >>But as long as the feature has isolated state, we can solve the
> >>problem robustly with subsections.
> >>
> >>Regards,
> >>
> >>Anthony Liguori
> >I see. I'm unhappy with the facts that
> >1. if (feature) is spread all over the code instead
> >    of just in migration
> 
> Yeah, but it's no different than say msi enablement.

I think it's different, some guests might have bad support for msi,
so ability to disable it in qemu is useful.

> >2. it is also obfuscated with if (flow_control)
> >    instead of plain if (migrate to qemu<  0.14)
> >    so removing it will be much harder
> >3. this forces anyone who wants
> >    a VM compatible with qemu 0.13 to also lose data,
> >    even if migration to 0.13 is never attempted.
> 
> It depends on whether we consider flow control a feature or a bug.
> If it's a bug, then we have to bump the live migration version and
> then live with the compatibility breakage.
> 
> Regards,
> 
> Anthony Liguori

I'm not sure. I don't see how it's useful to drop frames but maybe I'm wrong.
Amit?

> 
> >>>We also need API to add subsections without vmstate,
> >>>because virtio serial wasn't yet converted.
> >>>