cirros image fails to boot when libvirt.cpu_mode=custom and libvirt.cpu_model=pentium

[jiang, yunhong]

I try to set the libvirt.cpu_mode=custom and libvirt.cpu_model=pentium, and then boot a vm using cirros image. The 'nova boot' command create the instance successfully while the instance failed to boot because the image requires x86_64 while pentium has no x86_64 support.

Then I try to set the image's architecture property to be x86_64, with "glance image-update IMG-UUID --property architecture=x86_64". After this, the instance is created successfully by 'nova boot' but instance still fails to boot.

I think the reason is because, when libvirt_cpu_model=perntium, the compute node should in fact report it's capability as that of Pertium processor, instead of the host processor anymore.

[Joe Gordon (jogo)]

Without any log files and knowing which version this is on, this bug is difficult to triage.

[jiang, yunhong (yunhong-jiang)]

More information:

environment:
It's in the trunk of nova with libvirt virt driver. The hardware is latest Intel CPU feature with x86_64 support for sure.

a) nova conf:
[libvirt]
......
cpu_mode = custom
cpu_model=pentium
virt_type = kvm
...

b) mark the image as x86_64:
glance image-update 4d677ad0-47f5-4294-bb8e-d416735199c3 --property architecture=x86_64

c) start an VM

nova boot --image cirros-0.3.2-x86_64-uec --flavor m1.small 125

d) nova list:
the instance is up.
|3c5acf31-ebf9-48db-9d47-9d37e010e3dd | 125 | ACTIVE | - | Running | private=10.0.0.4 |

e) failed to ssh to the guest, but vnc to the guest show:
The guest OS failed to boot with information that "This kernel requires an x86-64 CPU, but only detected an i586 CPU.".

[Daniel Berrange (berrange)]

IMHO this is just user error - don't ask for such an outdated CPU model - use one of the sensible modern models - Nehlem, Haswell, SandyBridge, etc.

[jiang, yunhong (yunhong-jiang)]

I think the key point is, if we configure the host cpu_model, then we should expose the cpu_feature information of the cpu_model one, instead of the real host CPU features. So that compute_capability filter will not make wrong decision.

The pentium is just a extreme case. Considering some specific CPU features like SSE4.1. If host has SSE4.1 but the libvirt.cpu_model will disable it. The nova will still assign the host to a guest requesting SSE4.1 features.

[Sean Dague (sdague)]

Could be a feature for the safety check here