Comment # 9 on bug 93911 from David Breese

This did not fix the issue.  Still present in kernel 4.4 and 4.4.1, both of
which I believe contain the relevant commit mentioned in bug 91268.

No idea what this means or if it will be any use, but I notice that running
without "iommu=soft" on the kernel command line, suddenly the bug starts
occurring more than once an hour.  I have been running with "iommu=soft", in
which case the bug seems to happen more like once every 20 hours of X11 use.

Otherwise, mostly still the same package versions and dmesg as posted above,
but I can post the precise new info if desired.  Let me know what info I can
provide that would be useful.