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------- Comment From lagarcia@xxxxxxxxxx 2017-02-20 20:12 EDT-------
This is related to KVM guests memory configuration on IBM Power systems.

In the past we documented stuff like this in a wiki [0]. Not sure if
Canonical has a more appropriate place to document these
restrictions/workarounds.

0. https://wiki.ubuntu.com/ppc64el/uKVM

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https://bugs.launchpad.net/bugs/1666286

Title:
  Memory hot plug: increase the number of slots from '32' to '256'
  (Documentation)

Status in ubuntu-docs package in Ubuntu:
  Incomplete

Bug description:
  IBM would like to have the following documented wherever appropriate:

  ---Problem Description---
  In Ubuntu 17.04, when trying to increase the number of memory slots from 32 to 256, only 63 slots were available even though 256 slots was specified in the guest xml.

  Machine Type = POWER8

  Even though 256 slots are supported in QEMU 2.8, distributed with
  Ubuntu 17.04, there is an implicit limit coming from the vhost
  backend, limiting the  number of max memory slots that are supported.

  The work around is to increase the vhost mem slots by reloading the
  vhost module in the guest:

  [root@localhost ~]# modprobe -r vhost
  [root@localhost ~]# modprobe vhost max_mem_regions=512
  [root@localhost ~]# cat /sys/module/vhost/parameters/max_mem_regions
  512

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