Thanks for uncovering just another bug on the case (it is four now, so
we can already see that it is complex and comes back in different
aspects).

>From looking at the debdiffs a few formalities:
- origin, please point to some place you go this from "based on a patch by Len 
<lwh...@coreitx.com>" is nothing you can find, the comment might be useful but 
should be accompanied by an URL

In general it seems this shall become a patch we forever apply on top
(not upstream, needs to go to all releases now and in future), that kind
of change almost always is wrong.

If that would be the right solution then upstream would apply it. We
have seen too many cases where such a kind of forever-carry-on-delta
just makes us worse and worse. Now it might be bad in Xenial, but due to
that we can "infect" more and more with a bad behavior. E.g. when an
upstream change makes this subtly fail even worse and we only realize
later one.

Since the root cause seems to be like "... virtqueue got into an
inconsistent state in 2.5.0 and it carried over ..." (from
https://lists.gnu.org/archive/html/qemu-devel/2016-11/msg02830.html) I
wonder if we have to make a hard cut at some point instead of "carry
delta forever". More like "fix it in the old release and require a guest
restart before migration".

If you think applying that change to all releases is indeed the right
solution drive the upstream discussion and once applied there we can
backport SRU it to the existing releases. That also will provide a good
"origin" tag then as we can point at the commit in qemu git and drop it
once we merge a version with it.

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https://bugs.launchpad.net/bugs/1894772

Title:
  live migration of windows 2012 r2 instance with virtio balloon driver
  fails from mitaka to queens.

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