[Openstack-operators] Libvirt CPU map (host-model)

Paul Browne pfb29 at cam.ac.uk
Mon Oct 9 10:37:22 UTC 2017

Hello Belmiro,

We ran into this issue recently, similarly upgrading a RHEL7.3 OpenStack
Platform Overcloud to RHEL7.4 and in the process upgrading libvirtd.

For instances that were spawned prior to this upgrade, we see the CPU flags
[1] , but for new instance workload the CPU flags [2]. Notably the
CMT=disabled flag is present in [1] but absent in [2]

This similarly prevents live migration of the older spawned instances, as
the CMT=disabled flag is rejected.

A RH bugzilla [3] was opened on the issue which attracted a lot of really
good contributions from libvirt maintainers. The one sure-fire workaround
we'd found is just to cold-boot the instance again, starting it under the
new libvirtd. But from that BZ there is also a slightly more hack-ish
workaround to hand-edit the running domain XML and clear the offending CMT
flag (comment 12 on that BZ).

Hope this helps some,

Paul Browne

[1] https://pastebin.com/JshWi6i3
[2] https://pastebin.com/5b8cAanP
[3] https://bugzilla.redhat.com/show_bug.cgi?id=1495171

On 9 October 2017 at 04:59, Belmiro Moreira <
moreira.belmiro.email.lists at gmail.com> wrote:

> Hi,
> the CPU model that we expose to the guest VMs varies considering the
> compute node use case.
> We use "cpu_mode=host-passthrough" for the compute nodes that run batch
> processing VMs and "cpu_mode=host-model" for the compute nodes for service
> VMs. The reason to have "cpu_mode=host-model" is because we assumed that
> new CPUs (in the libvirt map) will continue to support previous features
> allowing for live migration when we need to move the VMs to a new CPU
> generation.
> We recently upgraded from CentOS7.3 (libvirt 2.0.0) to CentOS7.4 (libvirt
> 3.2.0) and noticed that now libvirt maps a slightly different CPU for the
> guests. For example, still "Haswell no-TSX" but no mention to the feature
> "cmt". This blocks suspended VMs to restore and live migrate.
> Has anyone experienced this same problem?
> We are thinking in few solutions but none of them are nice (downgrade
> libvirt? hard reboot instances? ...)
> thanks,
> Belmiro
> _______________________________________________
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> OpenStack-operators at lists.openstack.org
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Paul Browne
Research Computing Platforms
University Information Services
Roger Needham Building
JJ Thompson Avenue
University of Cambridge
United Kingdom
E-Mail: pfb29 at cam.ac.uk
Tel: 0044-1223-746548
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