Question about migrate the vm with host-passthrough cpu

[Kashyap Chamarthy]

[Catching up with libvirt-users list after a long while ... hence the
delay in my response.]

On Fri, Feb 26, 2021 at 03:58:21PM +0800, Guoyi Tu wrote:
> 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.

Just to reinforce Daniel's point below: 'host-passthrough' and live
migration is tricky ... so the safest suggestion is to go with identical
physical CPUs on source and destination hosts, along with identical
kernel, _and_ CPU microcode versions (see the performance counters point
below; and also thanks to Meltdown/Spectre).

Or else live migration is very much likely to fail.

> 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:

[...]

> > 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
> 
> 

-- 
/kashyap