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[Chris Friesen <chris.friesen@xxxxxxxxxxxxx>]

The code at https://review.openstack.org/#/c/534384/ has been merged,
and should allow the operator to explicitly add the pdpe1gb flag.

Marking as fixed.

** Changed in: nova
       Status: Confirmed => Fix Released

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https://bugs.launchpad.net/bugs/1538565

Title:
  Guest CPU does not support 1Gb hugepages with explicit models

Status in OpenStack Compute (nova):
  Fix Released

Bug description:
  The CPU flag pdpe1gb indicates that the CPU model supports 1 GB
  hugepages - without it, the Linux operating system refuses to allocate
  1 GB huge pages (and other things might go wrong if it did).

  Not all Intel CPU models support 1 GB huge pages, so the qemu options
  -cpu Haswell and -cpu Broadwell give you a vCPU that does not have the
  pdpe1gb flag. This is the correct thing to do, since the VM might be
  running on a Haswell that does not have 1GB huge pages.

  Problem is that Nova flavor extra specs with the libvirt driver for
  qemu/kvm only allow to define the CPU model, either an explicit model
  or "host". The host option means that all CPU flags in the host CPU
  are passed to the vCPU. However, the host option complicates VM
  migration since the CPU would change after migration.

  In conclusion, there is no good way to specify a CPU model that would
  imply the pdpe1gb flag.

  Huge pages are used eg with dpdk. They improve the performance of the
  VM mainly by reducing tlb size.

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