Hi Rolando,
we really should stick to here verify the issue that we fixed (the vvfat rw 
fail) and after you gave me a 50%-heartache :-) I verified proposed on my own 
now.

It just works via
1. echo "deb http://archive.ubuntu.com/ubuntu/ xenial-proposed restricted main 
multiverse universe" >> /etc/apt/sources.list
2. apt update
3. apt install qemu-block-extra qemu-kvm qemu-system-common qemu-system-x86 
qemu-utils

Now (re)-start your vvfat guest and it will work to write to the vvfat disk.
It did so for me at least.


I read through the issue you are seeing now and that is IMHO as a TL;DR saying 
"the vvfat emulation of a directory is not matching mounting it directly in the 
guest.
Am I right with that the summary of above is
1. create empty fat (not vvfat) image bering-own.qcow2
compare running
a) mount image to /mnt and append it as vvfat to the guest
vs
b) pass the qcow file itself as a drive to the guest
If I'm correct in reading what you do that is a totally different case.
If you look at the code [1] and some doc [2]+[3] you'll see that this driver is 
very restricted.
I'd never use it (or have seen it used) as root file system, not even for 
something critical other than simple data sharing between win guests and the 
host.
Quoting:
What you should never do:
- use non-ASCII filenames ;
- use "-snapshot" together with ":rw:" ;
- expect it to work when loadvm’ing ;
- write to the FAT directory on the host system while accessing it with the 
guest system.

There are slight improvements in [4], but also see the discussion [3]
which ended as Won't Fix.

Quoting from [3] is good as well: "vfat is a last-resort quick-n-dirty
fix for a problem when you have a guest which doesn't have any other
ways to communicate except of a fat image (read: a floppy).  In all
other cases just don't use it."

[1]: https://github.com/qemu/qemu/blob/master/block/vvfat.c
[2]: https://en.wikibooks.org/wiki/QEMU/Devices/Storage
[3]: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=419929
[4]: https://bugs.launchpad.net/qemu/+bug/1599539


All that said for the initially reported issue I can confirm that the bug is 
fixed and verified in proposed. Further insufficiencies of vfat would be 
another bug, but actually more upstream development work. If you open a new one 
I'm happy to discuss, but unlikely will find the time to do the related dev 
work.

** Bug watch added: Debian Bug tracker #419929
   http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=419929

** Tags removed: verification-needed
** Tags added: verification-done

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https://bugs.launchpad.net/bugs/1684239

Title:
  vvfat core dump when enabling RW

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