libvirt users - Feb 2021 - Question about migrate the vm with host-passthrough cpu

Hi there,

Sorry to bother you, but could I ask you a question about the live 
migration of virtual machine configured with host-passthough cpu, it 
confuse me a lot.

Here i have two hosts with different cpu, and the cpu feature set of old
host is subset of the new one (by virsh cpu-compare). If the vm was 
first started on the old host,is it safe to migrate the vm between the 
two hosts back and forth with the vm always keep running?

I've test the case in my environment, and the migration succeeds, the 
cpu family/model/stepping and features of lscpu in Guest OS is the same.


-- 
Best Regards,
Guoyi Tu

On Wed, Feb 24, 2021 at 09:42:37PM +0800, Guoyi Tu
wrote:
> Hi there,
> 
> Sorry to bother you, but could I ask you a question about the live
migration
> of virtual machine configured with host-passthough cpu, it confuse me a
lot.
> 
> Here i have two hosts with different cpu, and the cpu feature set of old
> host is subset of the new one (by virsh cpu-compare). If the vm was first
> started on the old host,is it safe to migrate the vm between the two hosts
> back and forth with the vm always keep running?
The CPUID features are the biggest compatibility issue that is likely
to cause problems generally, but I believe there can be others.

For example if performance counters are exposed to the guest, these
are likely to vary between different CPU models. Even different BIOS
settings can change performance counters exposed by a CPU (HT vs no-HT)

There might also be problems with CPU TSC frequency differences between
the hosts.
> I've test the case in my environment, and the migration succeeds, the
cpu
> family/model/stepping and features of lscpu in Guest OS is the same.
Right, the issue is that the migration appears to succeed from QEMU's
POV, but the guest may none the less see incompatibilities which
result in a crash or incorrect behaviour an arbitrary amount of time
later. It is really hard to guarantee that behaviour is going to be
correct in all scenarios.

Personally I would consider the use of host-passthrough to be unsupportable
in production environment, unless the CPUs are all homogeneous.

Regards,
Daniel
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