Re: [Qemu-devel] [PATCH RFC 1/8] virtio: add subsections to the migration stream

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

On Thu, May 15, 2014 at 11:58:25AM +0200, Andreas Färber wrote:
> Am 15.05.2014 11:52, schrieb Michael S. Tsirkin:
> > On Thu, May 15, 2014 at 11:20:18AM +0200, Andreas Färber wrote:
> >> Am 15.05.2014 09:04, schrieb Greg Kurz:
> >>> On Thu, 15 May 2014 12:16:35 +0530
> >>> Amit Shah <amit.shah@redhat.com> wrote:
> >>>> On (Thu) 15 May 2014 [09:23:51], Michael S. Tsirkin wrote:
> >>>>> On Thu, May 15, 2014 at 11:34:25AM +0530, Amit Shah wrote:
> >>>>>> On (Wed) 14 May 2014 [17:41:38], Greg Kurz wrote:
> >>>>>>> Since each virtio device is streamed in its own section, the idea is to
> >>>>>>> stream subsections between the end of the device section and the start
> >>>>>>> of the next sections. This allows an older QEMU to complain and exit
> >>>>>>> when fed with subsections:
> >>>>>>>
> >>>>>>> Unknown savevm section type 5
> >>>>>>> Error -22 while loading VM state
> >>>>>>
> >>>>>> Please make this configurable -- either via configure or device
> >>>>>> properties.  That avoids having to break existing configurations that
> >>>>>> work without this patch.
> >>
> >> Since backwards migration is not supported upstream, wouldn't it be
> >> easiest to just add support for the subsection marker and skipping to
> >> the end of section in that downstream?
> > 
> > Backwards and forwards migration need to be supported,
> > customers told us repeatedly. So some downstreams support this
> > and not supporting it upstream just means downstreams need
> > to do their own thing.
> > 
> > As importantly, ping-pong migration is the only
> > reliable way to stress migration.
> > 
> > So if we want to test cross-version we need it to work
> > both way.
> > 
> > Finally, the real issue and difficulty with cross-version migration is
> > making VM behave in a backwards compatible way.  Serializing in a
> > compatible way is a trivial problem, or would be if the code wasn't a
> > mess :) Once you do the hard part, breaking migration because of the
> > trivial serialization issue is just silly.  And special-casing forward
> > migration does not make code simpler, it really only leads to
> > proliferation of hacks and lack of symmetry.
> > 
> > So yes it's a useful feature, and no not supporting it does
> > not help anyway.
> 
> It seems you misunderstand. I was not saying it's not useful.
> 
> My point is that VMStateSubsections added in newer versions (and thus
> not existing in older versions) need to be handled for any
> VMState-converted devices. So why can't we make that work for virtio too?
> 
> Andreas

Sure.
AFAIK the way to this works is by adding an "field_exists" callback,
and not sending the section when we are in a compat mode.

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