Comment 0 for bug 1829555

When nested kvm virtualization is used (with host-passthrough), if the first level guest is a trusty vm, odd behavior is seen in the second level guest:

  host os:
  disco/5.0.0-15.16-generic/qemu 1:3.1+dfsg-2ubuntu3.1
  contents of /sys/devices/system/cpu/vulnerabilities/mds:
     Mitigation: Clear CPU buffers; SMT vulnerable

  1st level vm:
  trusty/4.4.0-148.174~14.04.1-generic/qemu 2.0.0+dfsg-2ubuntu1.46
  contents of /sys/devices/system/cpu/vulnerabilities/mds:
    Mitigation: Clear CPU buffers; SMT Host state unknown

  2nd level vm:
  bionic/4.15.0-50.54-generic
  contents of /sys/devices/system/cpu/vulnerabilities/mds:
    Not affected

This behavior is not seen when the first level guest is a xenial or bionic vm (same bare metal hardware):

  1st level vm:
  bionic/4.15.0-50.54-generic/qemu 1:2.11+dfsg-1ubuntu7.13
  contents of /sys/devices/system/cpu/vulnerabilities/mds:
    Mitigation: Clear CPU buffers; SMT Host state unknown

  2nd level vm:
  bionic/4.15.0-50.54-generic
  contents of /sys/devices/system/cpu/vulnerabilities/mds:
    Mitigation: Clear CPU buffers; SMT Host state unknown

and:

  1st level vm:
  xenial/4.4.0-148.174-generic/qemu 1:2.5+dfsg-5ubuntu10.39
  contents of /sys/devices/system/cpu/vulnerabilities/mds:
    Mitigation: Clear CPU buffers; SMT Host state unknown

  2nd level vm:
  bionic/4.15.0-50.54-generic
  contents of /sys/devices/system/cpu/vulnerabilities/mds:
    Mitigation: Clear CPU buffers; SMT Host state unknown

It's not clear whether this is an issue with linux/kvm or qemu in trusty.