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

[Yury Kotov]

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

>
> 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