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[Frank Heimes <1727331@xxxxxxxxxxxxxxxxxx>]

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

** Changed in: ubuntu-power-systems
       Status: Fix Committed => Fix Released

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https://bugs.launchpad.net/bugs/1727331

Title:
  NMI watchdog: BUG: soft lockup on Guest upon boot (KVM)

Status in The Ubuntu-power-systems project:
  Fix Released
Status in linux package in Ubuntu:
  Fix Released
Status in linux source package in Xenial:
  Fix Released

Bug description:
  == SRU Justification ==
  Xenial does not include the patch:
  "KVM: PPC: Book3S: Treat VTB as a per-subcore register, not per-thread",
  which is commit 88b02cf97bb7 in the upstream Linux kernel repository. The
  symptom of not having this patch is that guests running with threads > 1
  (that is, in SMT2, SMT4 or SMT8 mode) can give spurious soft-lockup messages 
  when they are not in fact locked up, if the guest kernel uses the VTB
  (virtual timebase) register in its softlockup detector code.

  IBM provided this backport of commit 88b02cf97bb7.

  == Fix ==
  commit 88b02cf97bb7e742db3e31671d54177e3e19fd89
  Author: Paul Mackerras <paulus@xxxxxxxxxx>
  Date:   Thu Sep 15 13:42:52 2016 +1000

      KVM: PPC: Book3S: Treat VTB as a per-subcore register, not per-
  thread

  == Regression Potential ==
  This patch is specific to powerpc.  It was provided by and tested by IBM.



  Environment:
  Host OS: Ubuntu 16.04 (4.4.0-97-generic)
  Guest OS: Ubuntu 16.04

  Host is running NovaLink and GPFS.  Guest gets filesystem from gpfs
  cluster.

  Issue:

  While booting the guest OS went into recovery.  dmesg shows CPU soft
  lockup.  This issue is easily recreatable when multiple VMs (about 10)
  are being started simultaneously.

  .......................................................................................................................

   - Paul Mackerras <pmac@xxxxxxxxxxx> - 2017-10-24 00:42:25 ==

  Looking at kernel sources:

  The Ubuntu host kernel (4.4.0-97) does not include the patch "KVM:
  PPC: Book3S: Treat VTB as a per-subcore register, not per-thread",
  which is commit 88b02cf97bb7 in the upstream Linux kernel repository.
  The symptom of not having this patch is that guests running with
  threads > 1 (that is, in SMT2, SMT4 or SMT8 mode) can give spurious
  soft-lockup messages when they are not in fact locked up, if the guest
  kernel uses the VTB (virtual timebase) register in its softlockup
  detector code.

  This is a backport of commit 88b02cf97bb7 from the upstream Linux
  kernel repository to the Ubuntu 4.4 kernel. It is the fix for the
  problem of seeing spurious soft lockup messages in guests running in
  an SMT mode greater than 1.

  These tests were run on a system with 20 cores.  Each VM had 1 core and 4 threads (SMT=4).
  It is working very well with this patch

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