Passing Flags from host machine to the Guest VM

Problem

Some CPUs do not pass the host machine CPU flags to the guest VMs by default. Which causes guest VMs unable to support that flags.

Environment

• Platform9 Managed OpenStack - v4.0 and Higher
• Host machine - model name : Intel(R) Xeon(R) Silver 4214R CPU @ 2.40GHz
• Guest VM - model name : QEMU Virtual CPU version 2.5+

Procedure

We need to pass the flags as extra parameters to resolve this issue. As shown in the below example guest VM was not getting avx flag from host, so we have passed the flag using cpu_model_extra_flags in "nova_override.conf".

flags : fpu de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pse36 clflush mmx fxsr sse sse2 syscall nx lm rep_good nopl xtopology eagerfpu pni cx16 x2apic hypervisor lahf_lm

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//deleted some flags 

flags : avx avx2 avx512f avx512dq rdseed adx smap clflushopt clwb avx512cd avx512bw avx512vl

Now, passing the flags in "/opt/pf9/etc/nova/conf.d/nova_override.conf" file as shown below:

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[libvirt]

cpu_mode = host-model

cpu_model_extra_flags = avx

After updating the file restart the pf9-ostackhost service and reboot the guest VM.

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# sudo systemctl restart pf9-ostackhost

# openstack server reboot --hard [Instance_UUID]

Now, guest VM will start showing that flags.