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[Qemu-devel] Re: new->old version migration
From: |
Michael S. Tsirkin |
Subject: |
[Qemu-devel] Re: new->old version migration |
Date: |
Tue, 8 Feb 2011 08:42:14 +0200 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
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.
> >
- [Qemu-devel] new->old version migration, Michael S. Tsirkin, 2011/02/07
- [Qemu-devel] Re: new->old version migration, Alex Williamson, 2011/02/07
- [Qemu-devel] Re: new->old version migration, Anthony Liguori, 2011/02/07
- [Qemu-devel] Re: new->old version migration, Michael S. Tsirkin, 2011/02/07
- [Qemu-devel] Re: new->old version migration, Anthony Liguori, 2011/02/07
- [Qemu-devel] Re: new->old version migration, Michael S. Tsirkin, 2011/02/07
- [Qemu-devel] Re: new->old version migration, Anthony Liguori, 2011/02/07
- [Qemu-devel] Re: new->old version migration,
Michael S. Tsirkin <=
- [Qemu-devel] Re: new->old version migration, Amit Shah, 2011/02/08
- Re: [Qemu-devel] Re: new->old version migration, Anthony Liguori, 2011/02/08
- Re: [Qemu-devel] Re: new->old version migration, Amit Shah, 2011/02/08
- Re: [Qemu-devel] Re: new->old version migration, Anthony Liguori, 2011/02/08
- Re: [Qemu-devel] Re: new->old version migration, Michael S. Tsirkin, 2011/02/08