[Bug 1926249] Re: postcopy migration fails in hirsute (solved)

[Felipe Alencastro]

Hi @ajkavanagh, this affects focal-hwe, jammy and will affect any new
releases unless this sysctl is set to 1.

** No longer affects: charm-nova-compute

** Also affects: qemu
   Importance: Undecided
       Status: New

** No longer affects: qemu

** Also affects: charm-nova-compute
   Importance: Undecided
       Status: New

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https://bugs.launchpad.net/bugs/1926249

Title:
  postcopy migration fails in hirsute (solved)

Status in OpenStack Nova Compute Charm:
  New
Status in qemu package in Ubuntu:
  Invalid

Bug description:
  FYI: this is an intended change, can be overwritten via config and
  this bug is mostly to have something puzzled users can find via search
  engines to explain and solve their issue.

  postcopy migration can in some cases be very useful
  => https://wiki.qemu.org/Features/PostCopyLiveMigration

  But with Hirsute kernel being 5.11 that now contains the following upstream 
change
  => 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=d0d4730ac2

  Due to that postcopy migration will fail like:

  + lxc exec testkvm-focal-from -- virsh migrate --unsafe --live --postcopy 
--postcopy-after-precopy kvmguest-focal-postcopy qemu+ssh://10.85.93.248/system
  error: internal error: unable to execute QEMU command 
'migrate-set-capabilities': Postcopy is not supported

  This will also apply to e.g. a Focal-HWE kernel once on v5.11 or to
  Focal userspaces in a container under a Hirsute kernel (that is the
  example above).

  This was done for security reasons, if you want/need to re-enable un-limited 
userfault handling to be able to use postcopy again you'd want/need to set the 
control knob to one like:
  $ sudo sysctl -w "vm.unprivileged_userfaultfd=1"

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