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[Dan Streetman <1915811@xxxxxxxxxxxxxxxxxx>]

** Also affects: cloud-archive
   Importance: Undecided
       Status: New

** Also affects: cloud-archive/stein
   Importance: Undecided
       Status: New

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https://bugs.launchpad.net/bugs/1915811

Title:
  Empty NUMA topology in machines with high number of CPUs

Status in Ubuntu Cloud Archive:
  New
Status in Ubuntu Cloud Archive stein series:
  New
Status in libvirt package in Ubuntu:
  Fix Released
Status in libvirt source package in Xenial:
  New
Status in libvirt source package in Bionic:
  New
Status in libvirt source package in Focal:
  New
Status in libvirt source package in Groovy:
  New

Bug description:
  [impact]

  libvirt fails to populate its NUMA topology when the machine has a
  large number of CPUs assigned to a single node. This happens when the
  number of CPUs fills the bitmask (all to one), hitting a workaround
  introduced to build the NUMA topology on machines that have non
  contiguous node ids. This has been already fixed upstream in the
  commits listed below.

  [scope]

  The fix is needed for Xenial, Bionic, Focal and Groovy.

  It's fixed upstream with commits 24d7d85208 and 551fb778f5 which are
  included in v6.8, so both are already in hirsute.

  [test case]

  On a machine like the EPYC 7702P, after setting the NUMA config to
  NPS1 (single node per processor), or just a VM with 128 CPUs, "virsh
  capabilities" does not show the NUMA topology:

  # virsh capabilities | xmllint --xpath '/capabilities/host/topology' -

  <topology>
        <cells num="0">
        </cells>
      </topology>

  When it should show (edited to shorten the description):

  <topology>
        <cells num="1">
          <cell id="0">
            <memory unit="KiB">5027820</memory>
            <pages unit="KiB" size="4">1256955</pages>
            <pages unit="KiB" size="2048">0</pages>
            <distances>
              <sibling id="0" value="10"/>
            </distances>
            <cpus num="128">
              <cpu id="0" socket_id="0" core_id="0" siblings="0"/>
              ....
              <cpu id="127" socket_id="127" core_id="0" siblings="127"/>
            </cpus>
          </cell>
        </cells>
      </topology>

  
  [Where problems could occur]

  Any regression would likely involve a misconstruction of the NUMA
  topology, in particular for machines with non contiguous node ids.

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