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. > >>>
