Re: [Qemu-devel] [PATCH 1/3] migration: Add x-validate-uuid capability

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Eric Blake (eblake@redhat.com) wrote:
> On 8/27/19 10:36 AM, Yury Kotov wrote:
> > 27.08.2019, 17:02, "Eric Blake" <eblake@redhat.com>:
> >> On 8/27/19 7:02 AM, Yury Kotov wrote:
> >>>  This capability realizes simple source validation by UUID.
> >>>  It's useful for live migration between hosts.
> >>>
> 
> >>
> >> Any reason why this is marked experimental? It seems useful enough that
> >> we could probably just add it as a fully-supported feature (dropping the
> >> x- prefix) - but I'll leave that up to the migration maintainers.
> >>
> > 
> > I thought that all new capabilities have x- prefix... May be it's really
> > unnecessary here, I'm not sure.
> 
> New features that need some testing or possible changes to behavior need
> x- to mark them as experimental, so we can make those changes without
> worrying about breaking back-compat.  But new features that are outright
> useful and presumably in their final form, with no further
> experimentation needed, can skip going through an x- phase.
> 
> > 
> >> In fact, do we even need this to be a tunable feature? Why not just
> >> always enable it? As long as the UUID is sent in a way that new->old
> >> doesn't break the old qemu from receiving the migration stream, and that
> >> old->new copes with UUID being absent, then new->new will always benefit
> >> from the additional safety check.
> >>
> > 
> > In such case we couldn't migrate from e.g. 4.2 to 3.1
> 
> I don't know the migration format enough to know if there is a way for
> 4.2 to unconditionally send a UUID as a subsection such that a receiving
> 3.1 will ignore the unknown subsection. If so, then you don't need a
> knob; if not, then you need something to say whether sending the
> subsection is safe (perhaps default on in new machine types, but default
> off for machine types that might still be migrated back to 3.1).  That's
> where I'm hoping the migration experts will chime in.

Right; the migration format can't ignore chunks of data; so it does need
to know somehow; the choice is either a capability or wiring it to the
machine type;  my preference is to wire it to the machine type; the
arguments are:
    a) Machine type
       Pro: libvirt doesn't need to do anything
       Con: It doesn't protect old machine types
            It's not really part of the guest state

    b) Capability
       Pro: Works on all machine types
       Con: Libvirt needs to enable it

So, no strong preference but I think I prefer (a).

Dave

> -- 
> Eric Blake, Principal Software Engineer
> Red Hat, Inc.           +1-919-301-3226
> Virtualization:  qemu.org | libvirt.org
> 



--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK