RE: [PATCH v1] target/i386: kvm: Block migration when enfore_cpuid is set to false

[Wang, Wei W]

On Thursday, July 11, 2024 10:14 PM, Daniel P. Berrangé wrote:
> On Thu, Jul 11, 2024 at 02:13:31PM +0000, Wang, Wei W wrote:
> > On Thursday, July 11, 2024 8:25 PM, Daniel P. Berrangé wrote:
> > > On Thu, Jul 11, 2024 at 12:10:34PM +0000, Wang, Wei W wrote:
> > > > On Thursday, July 11, 2024 7:48 PM, Daniel P. Berrangé wrote:
> > > > > On Wed, Jul 03, 2024 at 10:49:12PM +0800, Wei Wang wrote:
> > > > AFAIK, many users are not aware of this, and also we couldn't
> > > > assume everybody knows it. That's why we want to add the enforcement.
> > >
> > > Users who directly launch QEMU are expected to know about QEMU
> > > config details for migration. If they don't, then they ought to be
> > > using a higher level tool like libvirt, which ensures the configuration is
> migration compatible.
> >
> > Agree that libvirt has this advantage and is more user friendly. But
> > it doesn't seem to solve the issue mentioned by this patch - if users don't
> explicitly set "enforce=true"
> > in libvirt configs for the guest, then migrating the guest across
> > hosts with different features could still be risky. Unless there is
> > similar enforcement in libvirt to require users to set "enforce=true" for 
> > the
> guest to be migratable.
> 
> Yes, libvirt takes steps to ensure CPU compatibility before migrating.

Thanks for sharing, but curious about those steps. For example, with 
"enforce=off"
(by default), features on the destination could be filtered by QEMU (kind of 
silently,
just has warning logs).
How would the source side libvirt get informed about more features getting 
filtered by
the destination side QEMU (as the destination host has less support for this 
vcpu model)?
This causes inconsistencies.