Question about migrate the vm with host-passthrough cpu

[Guoyi Tu]

Thanks for you reply.

I checked the qemu code, the destination qemu instance actually exposed
the cpu feature of new host , and if execute the cpuid instruction in
Guest OS after migration the family/model/stepping is the same as new
host.

On Thu, 2021-02-25 at 14:10 +0000, Daniel P. Berrangé wrote:
> 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