But fear not: Support for CPU models in KVM can prevent migration in cases where compatibility issues are encountered by possibly checking all features in use on the source system against supported features on the target system. If the target system is lacking support for a feature that is strictly required for a KVM guest to run, the host aborts the migration with a respective error message - any attempt to migrate Guest B to the zEC12 would be rejected, and Guest B would continue to run on z14 instead.
While this is certainly a useful feature to prevent havoc from happening, the
administrator is still left with a guest that cannot be migrated to the desired target system. Therefore, a bit of care is required when defining guests.
There is a way out: If we define Guest B on the z14 using a zEC12 CPU model, only the back-level machine's CPU features will be required to run Guest B, and we can guarantee live guest migration capability among all models of at least the zEC12 CPU model generation!
To choose a CPU model, pick any of the following four base modes:
- Safe settings
- Host model: Expanded to a reproducible model
<cpu mode='host-model'/> - Custom (variant 1): Choose a pre-defined model
<cpu mode='custom'>
<model>zEC12</model>
</cpu> - Expert Settings
- Host passthrough: Also prevents any CPU model checks prior to live guest migration, and is therefore discouraged.
<cpu mode='host-passthrough'/> - Custom (variant 2): Modify a pre-defined model
<cpu mode='custom'>
<model>z13-base</model>
<feature policy='disable' name='dfp'/>
<feature policy='require' name='vx'/>
</cpu>
If not specified, KVM defaults to host-passthrough. Because it disables CPU model checks, we recommend to always specify host-model instead: It enables the same CPU model features that host-passthrough would, but provides CPU model checks prior to any live guest migration.
Note that not all Linux distributions providing KVM on Z support CPU models, e.g. SLES 12 SP3 and Ubuntu 16.04. Besides the obvious lack of protection against migration faults and the flexibility provided by CPU models, live guest migration from a KVM host with CPU model support to these distributions will not work! Hence, migration from SLES 12 SP4 (which does provide CPU model support, see here) to SLES 12 SP3 does not work.
Use virsh domcapabilities to identify a host's CPU model as well as all compatible models:
$ virsh domcapabilities
<domainCapabilities>
[...]
<cpu>
<mode name='host-passthrough' supported='yes'/>
<mode name='host-model' supported='yes'>
<model fallback='forbid'>z13.2-base</model>
<feature policy='require' name='aen'/>
[...]
</mode>
<mode name='custom' supported='yes'>
<model usable='yes'>z890.2</model>
<model usable='yes'>z990.4</model>
<model usable='no'>z14ZR1</model>
<model usable='yes'>z10BC.2</model>
<model usable='yes'>z196.2</model>
<model usable='no'>z14</model>
<model usable='yes'>z9BC-base</model>
[...]
The respective XML snippets can be copied directly into guest domain definitions.
A comprehensive overview of all available CPU models can be retrieved from QEMU as follows:
$ qemu-system-s390x -cpu help -enable-kvm
[...]
s390 z196-base IBM zEnterprise 196 GA1 (static, migration-safe)
s390 z196 IBM zEnterprise 196 GA1 (migration-safe)
s390 z196.2-base IBM zEnterprise 196 GA2 (static, migration-safe)
s390 z196.2 IBM zEnterprise 196 GA2 (migration-safe)
s390 z114-base IBM zEnterprise 114 GA1 (static, migration-safe)
s390 z114 IBM zEnterprise 114 GA1 (migration-safe)
s390 zEC12-base IBM zEnterprise EC12 GA1 (static, migration-safe)
s390 zEC12 IBM zEnterprise EC12 GA1 (migration-safe)
s390 zEC12.2-base IBM zEnterprise EC12 GA2 (static, migration-safe)
s390 zEC12.2 IBM zEnterprise EC12 GA2 (migration-safe)
s390 zBC12-base IBM zEnterprise BC12 GA1 (static, migration-safe)
s390 zBC12 IBM zEnterprise BC12 GA1 (migration-safe)
s390 z13-base IBM z13 GA1 (static, migration-safe)
s390 z13 IBM z13 GA1 (migration-safe)
s390 z13.2-base IBM z13 GA2 (static, migration-safe)
s390 z13.2 IBM z13 GA2 (migration-safe)
s390 z13s-base IBM z13s GA1 (static, migration-safe)
s390 z13s IBM z13s GA1 (migration-safe)
s390 z14-base IBM z14 GA1 (static, migration-safe)
s390 z14 IBM z14 GA1 (migration-safe)
The models *-base models, also described with keyword static, represent a base version of the respective machine generation without any additional features set. This is in contrast to the non-base versions, which will can add further features on top.
As some general guidance, here is a simplified table illustrating the migration compatibility between IBM Z machine generations:
| zEC12 | z13 | z14 | z14 ZR1 | |
|---|---|---|---|---|
| zEC12 | - | no | no | no |
| z13 | yes | - | no | no |
| z14 | yes | yes | - | yes |
| z14 ZR1 | yes | yes | yes | - |
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