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Public bug reported:

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

** Affects: ubuntu-docs (Ubuntu)
     Importance: Undecided
     Assignee: Taco Screen team (taco-screen-team)
         Status: New


** Tags: architecture-ppc64le bugnameltc-151531 severity-medium targetmilestone-inin1704

** Tags added: architecture-ppc64le bugnameltc-151531 severity-medium
targetmilestone-inin1704

** Changed in: ubuntu
     Assignee: (unassigned) => Taco Screen team (taco-screen-team)

** Package changed: ubuntu => ubuntu-docs (Ubuntu)

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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:
  New

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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