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