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[Bug 1549494] Re: arm64: guest hangs when ntpd is running


** Changed in: linux (Ubuntu Xenial)
       Status: In Progress => Fix Committed

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  arm64: guest hangs when ntpd is running

Status in linux package in Ubuntu:
  Fix Committed
Status in linux source package in Xenial:
  Fix Committed

Bug description:
  SRU Justification:

  Impact: KVM guests hang on arm64 host running ntpd

  Fix: From linux-next:
  commit 1c5631c73fc2261a5df64a72c155cb53dcdc0c45
  Author: Marc Zyngier <marc.zyngier@xxxxxxx>
  Date:   Wed Apr 6 09:37:22 2016 +0100

      KVM: arm/arm64: Handle forward time correction gracefully
      BugLink: http://bugs.launchpad.net/bugs/1549494
      On a host that runs NTP, corrections can have a direct impact on
      the background timer that we program on the behalf of a vcpu.
      In particular, NTP performing a forward correction will result in
      a timer expiring sooner than expected from a guest point of view.
      Not a big deal, we kick the vcpu anyway.
      But on wake-up, the vcpu thread is going to perform a check to
      find out whether or not it should block. And at that point, the
      timer check is going to say "timer has not expired yet, go back
      to sleep". This results in the timer event being lost forever.
      There are multiple ways to handle this. One would be record that
      the timer has expired and let kvm_cpu_has_pending_timer return
      true in that case, but that would be fairly invasive. Another is
      to check for the "short sleep" condition in the hrtimer callback,
      and restart the timer for the remaining time when the condition
      is detected.
      This patch implements the latter, with a bit of refactoring in
      order to avoid too much code duplication.

  A) Install ntp on the host (I'm using a 2 socket ThunderX)
  B) Boot up a large VM (95 CPUs/4G memory)
  C) Within the VM, rebuild the Ubuntu kernel package
  If it doesn't hang - bug is gone.

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