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

[Dr. David Alan Gilbert]

* Yury Kotov (address@hidden) wrote:
> 03.09.2019, 14:25, "Dr. David Alan Gilbert" <address@hidden>:
> > * Eric Blake (address@hidden) wrote:
> >>  On 8/27/19 10:36 AM, Yury Kotov wrote:
> >>  > 27.08.2019, 17:02, "Eric Blake" <address@hidden>:
> >>  >> 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).
> 
> IIUC the (a) option requires to add a piece of code to every machine type.
> This is much more complicated than adding a capability.

Actually it doesn't - you just add a property, default it to true and
then add an entry in hw_compat_4_1 to turn it off for older types.

> If you don't mind, I suggest to keep the current version.

That's OK.

Dave

> >
> > Dave
> >
> >>  --
> >>  Eric Blake, Principal Software Engineer
> >>  Red Hat, Inc. +1-919-301-3226
> >>  Virtualization: qemu.org | libvirt.org
> >
> > --
> > Dr. David Alan Gilbert / address@hidden / Manchester, UK
> 
> Regards,
> Yury
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK