Cluster CPU Model setting - EVC like setting

[jayst136]

i noticed my cluster is defaulting to CPU model "host passthrough". 

On my nodes,  "virsh capabilities | grep -i model" shows:
<model>Skylake-Server-IBRS</model>

I don't know what this configuration means for the CPU features presented. What does "host-passthrough" actually do?

[u34666]

This is correct, the default cluster setting is "host passthrough".

My understanding is: host-passthrough, allows virtual machines to utilize the native capabilities of the underlying host CPU at power-on, without applying any CPU feature masking.

In your case the VM would see Skylake-Server-IBRS CPU features. 

[jayst136]

that would make sense but i read a few other posts here that contradict that in my opinion. What would this setting do when i have 2 types of CPU's in my cluster for example?

[u34666]

The lowest value (oldest generation). Otherwise you can't "Live Migrate" a VM that has been started on a host with a newer generation to a host that has an older generation.

[Arnout_Verbeken · Accepted solution]

We use host-passthrough in combination wiht "migratable=on".  You should also see this setting in your xml.

Purpose: When migratable='on' is set, libvirt ensures that the CPU configuration exposed to the guest VM only includes CPU features that are migratable. This means it excludes CPU features that are specific to the host's hardware and might not be available on other hosts in a cluster.

If your VM does not have this specific "migratable=on", it means it was created long time ago on older versions.  You can fix that with:

• power down the vm
• migrate the vm to other host (with reconfigure)
• power the vm on

(same as with Vmware if you want to enable EVC on existing VM's.  The migrate to other host is needed so the xml is recreated.)