[Bug 1748280] Re: 'nova reboot' on arm64 is just 'nova reboot --hard' but slower

[Balint Reczey]

** Branch linked: lp:~rbalint/britney/u-u-arm64-broken-reboot-xenial

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https://bugs.launchpad.net/bugs/1748280

Title:
  'nova reboot' on arm64 is just 'nova reboot --hard' but slower

Status in nova package in Ubuntu:
  New

Bug description:
  Within Canonical's multi-architecture compute cloud, I observed that
  arm64 instances were taking an unreasonable amount of time to return
  from a 'nova reboot' (2 minutes +, vs. ~16s for POWER in the same
  cloud region).

  Digging into this, I find that a nova soft reboot request is doing
  nothing at all on arm64 (no events visible within the qemu guest, and
  no reboot is triggered within the guest), and then after some sort of
  timeout (~2m), 'nova reboot' falls back to a hard reset of the guest.

  Since this is entirely predictable on the arm64 platform (because
  there's no implementation of ACPI plumbed through), 'nova reboot' on
  arm64 should just skip straight to the hard reboot and save everyone
  some clock time.

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