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[Bug 1427075] Re: Stopping and starting KVM partitions results in guest kernel softlockup


** Changed in: linux (Ubuntu)
       Status: Confirmed => Fix Committed

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  Stopping and starting KVM partitions results in guest kernel

Status in linux package in Ubuntu:
  Fix Committed

Bug description:
  Stop/continue via qemu monitor can easily trigger soft-lockups due to incorrect VM timekeeping.

  [Test Case]
  On a single CPU KVM guest:
  yes > /dev/null &

  Followed by
  (qemu) stop

  Wait a while, then:
  (qemu) cont

  The following commits upstream:
  commit 545a2bf742fb41f17d03486dd8a8c74ad511dec2
  commit 4be1b29795d692d512bb67b770665d6f8ea5cb0b


  == Comment: #0 - Cyril Bur <cyrilbur@xxxxxxxxxxx> - 2015-02-23 18:03:41 ==
  +++ This bug was initially created as a clone of Bug #108455 +++

  I was investigating the cause of some ppc64le KVM guest softlockup
  warnings. On a single CPU KVM guest, I ran something to keep the guest

  yes > /dev/null &

  Followed by

  (qemu) stop

  Wait a while, then:

  (qemu) cont

  We get a softlockup error:

  BUG: soft lockup - CPU#0 stuck for 9220s! [yes:2389]


  I was going to file it away in the "don't do that" bin, but I notice
  x86 have something to detect a paused VM and avoid spewing the soft
  lockup error. Do we need something like this on ppc64?

  commit 5d1c0f4a80a6df73395fb3fc2c302510f8f09d36
  Author: Eric B Munson <emunson@xxxxxxxxx>
  Date:   Sat Mar 10 14:37:28 2012 -0500

      watchdog: add check for suspended vm in softlockup detector

      A suspended VM can cause spurious soft lockup warnings.  To avoid these, the
      watchdog now checks if the kernel knows it was stopped by the host and skips
      the warning if so.  When the watchdog is reset successfully, clear the guest
      paused flag.

  == Comment: #1 - Cyril Bur <cyrilbur@xxxxxxxxxxx> - 2015-02-23 18:03:55 ==

  I have been working on a fix for guest kernels. This requires two


  commit 545a2bf742fb41f17d03486dd8a8c74ad511dec2
  Author: Cyril Bur <cyrilbur@xxxxxxxxx>
  Date:   Thu Feb 12 15:01:24 2015 -0800

      kernel/sched/clock.c: add another clock for use with the soft
  lockup watchdog

  and 2/2

  commit 4be1b29795d692d512bb67b770665d6f8ea5cb0b
  Author: Cyril Bur <cyrilbur@xxxxxxxxx>
  Date:   Thu Feb 12 15:01:28 2015 -0800

      powerpc: add running_clock for powerpc to prevent spurious
  softlockup warnings

  Both are in upstream.

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