Regression warning about 4.4.17+18 / 4.6.7 / 4.7+ and Xen

[Stefan Bader]

There has been a recent (maybe not that recent, it was in 4.7) kernel regression
for Xen which made its way back to several stable trees[1]:

  xenbus: don't BUG() on user mode induced condition

It appears there is a fix on patchworks[2] but as of today was not included, yet
in upstream Linux.

For Yakkety the regression is part of the 4.6.0-11 unstable package. For Xenial
it is part of the pending next SRU cycle (master-next). I did a quick test with
the proposed patch on top of 4.6 in yakkety/master-next and it allowed me to
starts VMs again. Not sure whether we want to pick that additional patch or back
out the stable change introducing the problem. But either way the master-next
branches of both Xenial and Yakkety will cause a Xen regression as they are now.

The effect is that tools like xenstore-ls or xenstore-write no longer work and
it is no longer possible to start any VMs.

-Stefan

[1] https://lists.xenproject.org/archives/html/xen-devel/2016-08/msg02496.html
[2] https://patchwork.kernel.org/patch/9281193/

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