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

[Daniel P . Berrangé]

On Thu, Jul 11, 2024 at 03:09:28PM +0000, Wang, Wei W wrote:
> 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.

Libvirt queries QEMU to ask it what features were actually active
in the guest.

With regards,
Daniel
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